Note: these mistakes cannot be caught by spell-check

She felt accepted by everyone in the family, except her mother-in-law.

Of course the strychnine affected him. The effects of that poison are deadly.

In other words, affect is a verb and effect is a noun.

Note, however, that both words have secondary meanings that reverse this rule. Affect as a noun is also a psychological term meaning the feeling or emotion associated with a particular person, place, or object; effect as a verb is a highly formal (verging on arch) way of saying make. These usages should be far less common than those above:

Because Janis heard her mother sing that song every day during her childhood, it carried an unusual degree of affect for her.

By lowering the interest rates in December, the Federal Reserve was attempting to effect a change in people’s spending habits.

She wondered aloud if she would be allowed to compete in the spelling bee.

The criminal to whom I alluded yesterday eluded capture for another two weeks before an informant turned him in.

In a recent interview, Penn and Teller made an allusion to a famous illusion Harry Houdini had performed in the 1920s.

“Keep these screws apart from the others,” said Jim, as he fitted the Ikea bookcase together. “We’ll need them before we’re done, and nothing is more frustrating when you build these things than having to hunt around looking for a part.”

In other words, a part is an article plus a noun and means a piece of a whole; apart is an adjective meaning away from or separate. A part is usually followed by of; apart is usually followed by from.

Our dogs are both getting old.

The study shows no correlation, let alone a causal relationship, between wearing casual clothing to school and disciplinary problems.

In announcing the site for the construction of the new city hospital, the mayor cited the environmental impact study he had commissioned the year before. You can find the study on the city website, but if you quote it in your paper, be sure to cite it properly.

Her clothes were made out of the finest silk and linen. She had sewn some of them herself from some old tablecloths she found in her attic. The cloth from which they were made came from China.

“Henry,” said the head chef, “this balsamic reduction complements the fruit in this dessert remarkably well.”
Henry stammered his thanks but was too nervous to say more. He was not used to compliments from his boss, and he half-expected Monsieur Robert to follow up with a sarcastic aside.

In other words, complement means to complete, or, more casually, to accompany in a favorable way, whereas compliment means to say nice things about.

Consciously or not, Terry was taking advantage of Pandora when he asked her to stay home with him that night instead of going to the concert. She had a guilty conscience, though it was hardly her fault that he had broken his leg and been knocked unconscious, even if she had invited him on the ski trip. He was the one who insisted on skiing a double-black-diamond trail. She had always been too conscientious for her own good.

The conscience (a noun) is the part of our minds that makes us feel guilty or responsible, that keeps us from acting badly or unfairly; conscientious is the adjective form, used to describe someone with a highly developed conscience. Conscious (an adjective) refers to being aware or awake; consciousness is the noun form.

Defuse/Diffuse

You had better defuse that bomb quickly, or it will diffuse our bodies over a wide area.

She was definitely angry; she shook her fist at him and said defiantly, “I have lived here my entire life, and I will not sell you this land for any amount of money!”

This is actually a spelling error because these words should not be pronounced remotely alike: DEF-in-it-lee vs. de-FI [long i, as in TIE]-unt-lee.

Even in Dorothea’s old age, the smell of hibiscus would elicit memories of Tahiti, Philippe, and their passionate, illicit affair in the summer of her twenty-first year.

I could have, I should have, and I would have, but I didn’t and I’m sorry. [Not “I could of, I should of, I would of,” and so on.]

Authors writing in dialect — Mark Twain, Flannery O’Connor, John Kennedy Toole, and the like — sometimes break this rule.

A vast horde of adventurers, thieves, and mountebanks invaded the city in search of the hoard of treasure they had heard was hidden there.

It’s trying to find its mother.

Thus, it’s is a contraction of it is; its is a possessive meaning belonging to it. Just as you should not put an apostrophe in his or hers, you should not put one it its when you are using it as a possessive. Note that no such word as its’ exists.

She felt as if she needed to lie down. She just wanted to lay her head on the pillow and go to sleep. But she lay in bed for hours, and nothing happened. It was not until her sister finally called her that afternoon that she laid her worries aside.

Lay is a transitive verb, which means it takes a direct object. A gambler can lay his cards on the table, and a boxer can lay an opponent out with one punch, but one does not lay down.

Lie is an intransitive verb, which means it cannot take a direct object. You can lie down, or lie in bed, but you cannot lie something. What gets confusing is that the past tense of lie is also lay, whereas the past tense of lay is laid.

I led my daughter to the counter, where she bought ink for her pen and lead for her mechanical pencil. I wonder where her interest in writing will lead her?

The most common problem here is writing lead when you mean led.

Let’s go to the theatre on 4th Street. My uncle is the manager there and he lets me in for free.

The first is a contraction of the verb let and the pronoun us; the second is just the third-person singular form of the verb (meaning allow).

Bobby was loath to take Algebra II his sophomore year in high school; he liked mathematics well enough, but the teacher, Mr. Giddens, coached wrestling, and Bobby loathed him.

The first is an adjective meaning reluctant; the second is a verb meaning despise.

That key-chain is so loose that I am afraid you might lose your keys.

Note: The former is usually an adjective; the latter is a verb, but the same rule applies to the other forms of the words:

I saw Mr. Burns loosing his dogs on Bart. Losing his money didn’t make him any less mean.

As Ben picked out a pair of even baggier jeans, his father said, “Wearing your pants any looser than the ones you have now will make you look like a loser.”

There may be a good reason they never showed up. Maybe they missed their flight.

The first is a verb phrase; the second is an adverb.

“Mother, I just want to look at the mountains. Those peaks are spectacular,” said Hermione as she peeked through the window blinds. But it was really Lars, the ski instructor, who had piqued her interest.

Prejudice always infuriated Dr. Prendergast, no matter where or in whom he encountered it, perhaps because he grew up in a deeply prejudiced family.

He was entirely too ill to raise his head from the pillow, let alone rise to greet his guests.

As with lay and lie, the difference here is between a transitive and an intransitive verb. Raise is transitive, which means it takes a direct object, while rise is intransitive and does not.

No matter how her counselors attempted to rein her in, the people suffered under her reign of terror.

Reins are what you use to control horses, so as a verb rein means to control, or more specifically to steer or stop. Thus, one can loosen the reins, keep a tight rein, and so on. Reign means to rule and refers specifically to royalty, though sometimes the word is used metaphorically for anyone in a position of power.

Do you suppose he will accept the paper late? It was supposed to be handed in Monday.

She used to like Mumford and Sons more than Bon Iver; then she changed her mind.

They’re going over there to visit their relatives.

The pronunciation of the third word should be different from that of the first two, something like THAY-R instead of THAIR — that is, the vowel sound is longer in they’re.

One should not be surprised when a despot who has seized the throne is himself overthrown.

I went to Prague with two friends. We stopped in Budapest and Vienna, too.

I used to play that piano every day; now I just use it as a table.

She had always been wary of growing dependent on Tessa’s friendship, but now she was too weary to resist her offer.

Wary is etymologically related to beware and means to feeling trepidation about possible danger, while weary means tired.

Whether the weather is good or not, the wedding will be Saturday.

We’re going to visit Westminster Abbey, where you were yesterday.

The pronunciation of these words should be different, something like WUR, WEER, and either WAIR or, more traditionally, HWAIR (the wh combination, as Stewie Griffin knows, was long pronounced like hw and still is in some places).

You’re going to ruin your ears if you keep blasting your car stereo at full volume.

A lot of people like jazz. My friend Thelonius likes it a lot.

Note that many and much or considerable are usually preferable: Many people like jazz. My friend Thelonius likes it very much.

Spellcheck will not catch allot because it is a different word (meaning to allocate or parcel out)

Is that all right with you?

Note: alright appeared a mere seventy years after all right, so some style guides argue that it should be acceptable. More, however, still do not, and even the ones that do usually list all right as preferable. (Of course, in dialect the rules are different; you might even see a’ight in dialect.)

Have you already set the table?

This only applies to already used as an adverb; in a phrase in which all refers to a group and ready functions as an adjective, the words should not be combined: “Are your friends all done with finals? Mine are all ready for vacation.”

Other words that people frequently misuse

Use like to compare words or phrases; use as (or sometimes as if or as though) to compare clauses. Examples:

The only enjoys sports Ben enjoys are those that do not employ a clock, like tennis and baseball.
Hugh received her kiss like a prisoner being served his last meal: he knew he should savor it, but the dread of what would follow outweighed the pleasure of the moment.
Sarah knew her English degree would serve her well in any profession that depends on persuasion, like law.
Sarah knew her English degree would serve her well in any profession that depends on persuasion, as lawyers must.
Despite her in-laws’ rudeness, Jennifer did not lose her temper, as I would have.

The difference between these words is the degree of interruption. Continuous usually refers to physical description and implies no interruption:

His eyebrows grew so close together that they formed one continuous line across his face.

Continual usually refers to time and implies something repetitive to the point it becomes virtually constant:

Little Ethan’s continual banging of his toy drum had given Sarah a headache.

Repeated means something that happens again and again, but with intervals:

Steve became a successful attorney, despite his repeated failure to pass the bar exam.

Note that these rules also apply to the adverbial forms of the words (continuously/continually/repeatedly).

These words cannot serve as all-purpose replacements for describe, because both apply the visual nature of description to complex phenomena. You can depict the chaos of war, but you cannot depict a tree or a house. Similarly, an author can portray the struggle of the poor to rise in society — and, in a slightly different use of the word, an actor can portray Thomas Jefferson as a young man — but no one portrays Thanksgiving dinner (unless they portray it in a symbolic way, such as a baseball game portrayed as a pagan pastoral ritual).

The proper preposition to accompany this word is in most cases from, not than:

The reasons for Steerforth’s apparent kindness are different from what David Copperfield supposes.

To be disinterested means to be neutral or unbiased; a disinterested observer is usually a good thing, because that is someone who can view whatever is occurring fairly.

To be uninterested means to have no interest in something.

The reason for this confusion is not the two prefixes (dis- and un-) but two different meanings of the word interest: 1) the condition of wanting to know more about something, and 2) a benefit, stake, or involvement in something, as in the phrase a vested interest. Disinterested derives from definition #2; uninterested from definition #1.

Empathy is an understanding of someone else’s perspective and internal logic. Sympathy is agreement with or acceptance of someone else’s perspective and internal logic. You empathize with people if you understand why they feel the way they do. You sympathize with them if you feel the same way, or at least accept their response as legitimate. Empathy is paradoxically often more difficult for people than sympathy.

Literally, these words mean to call out of and to call upon. Evoke is usually used to describe how a stimulus of some kind causes an emotional or personal reaction:

This photograph of Sarah playing with her twin brother when they were seven always evokes happy memories of the house on Stanton Street.

Lord Byron’s “Darkness” evokes vivid images of destruction and feelings of horror in me.

Invoke, on the other hand, describes some sort of call for help, usually an appeal to a higher authority:

Elihu Root surprised the justices by invoking the First Amendment in his summation.

As he spoke the twisted words from the ancient text, all present realized what he intended: to invoke the aid of Cthulu and all his dark minions. And then they knew his lust for power had finally driven him mad.

Human has traditionally been an adjective, not a noun. Thus, the phrase is human being, meaning a being who is human. However, an equally acceptable (and shorter and simpler) term for an individual human being is person, and people is a perfectly fine word for a group of individuals. Human does work in the scientific sense, when one is considering homo sapiens sapiens as a species, as for example in an anthropology essay.

While one can find an occasional past use of human as a substitute for person, the growing use of humans for people in general seems to have developed out of science fiction. Having a nasty alien race call human beings people makes them seem less nasty, especially when you can have them spondaically (meaning with equally stressed syllables in the word) grunt: “KILL ALL the HYOO-MAHNZ.” But the conventions of sci-fi should not have such power over the language, so unless you can prove Klingon ancestry or have some particular reason for emphasizing our species (rarely the case), choose between person/people and human being(s).

A speaker or writer implies; the listener or reader infers. For example:

The senator arose from his desk and gravely addressed the reporters, “I don’t know what you are implying, gentlemen, but I infer from your tone that you think I have been dishonest, and I must tell you, sirs, I do not like that implication. Who knows what inference the voters may take from it?”

Conventionally a noun:

What impact do you think the plant closing will have on the economy here?

Although nouns can occasionally be used as verbs to create more vivid language, impact should not be considered a general replacement for affect. Using it this way often sounds ridiculous:

The high temperatures will impact the runners strongly today. 

Impact is vastly overused these days.

This word is often hyperbole. Few things are actually opposite, and the chances go down the more complex they are. Above all, people (including literary characters) are never opposite. Example:

Laertes is the opposite of Hamlet.

While these two characters have certain traits that are different, they also have much in common, and this will virtually always be true of any two characters. Also, never modify this word with adjectives such as complete or total or adverbial intensifiers like very. It is an absolute, and should never be intensified.

This is the proper adjective to describe something that prevents something from happening — no need for the more recent coinage (common in ads for new medicines and car maintenance) preventative.

Quote is properly a verb:

He always quotes Oscar Wilde when he wants to sound witty.

Quote is slang as a noun; the correct noun form is quotation. Unless you are hanging around a newsroom or starring in a production of The Front Page, don’t say something like, “This is a revealing quote.” 

In literary essays, even quotation does not usually make sense. A passage you quote is not a quotation until you quote it, so saying, “In this quotation” is not a meaningful way to set up your analysis of a quotation. Depending on the context, words such as passage, lines, speech, stanza, or paragraph will be better options.

This is the proper word when you want to create a transition that means in any event or whatever the case; the word irregardless — which most sources do not consider a proper English word — is either redundant or self-contradictory.

An extraordinarily weak word, especially when used in the passive voice:

Dickinson’s ideas about nature are related to those of Emerson and the English Romantics like Keats.

The problem is many kinds of relationships exist: if A and B are alike, they have a relationship, but if A and B are opposite, they also have a relationship. If A causes B they have a relationship, but a different relationship than if B causes A, or if both are caused by C. Relate fails to make the relationship clear. Find the appropriate verb to describe the relationship instead.

Similarly, do not just say that the reader or the audience can relate to a character. Do you mean sympathize with? Empathize with? Identify with? Admire? In this case, find the appropriate verb to describe the emotion. Avoid the word relatable, which has become common lately but which literally makes no sense in most circumstances: usually, one is not relating the thing being described to something else but relating oneself to the thing being described, so the ability to relate is one’s own, not the thing’s. Confused? Just avoid the word!

Simplistic is not a fancy way of saying simple. Simplistic is pejorative — an insult — whereas simple is neutral and can even be complimentary. If you say something is simplistic, you mean that it is shallow, unsophisticated, misleading, even stupid. If you say something is simple, you mean it is not complicated, and that can be a good thing. For example:

Your analysis of the Protestant Reformation is simplistic.

If your history professor writes that on your paper, you can expect a poor grade.

All of the songs are based on simple medieval melodies.

This may well be a line from a four-star review.

This word means all the way through and frequently causes two problems. First, people use it when it does not apply, for example when something only happens for part of the time, or stops happening after a certain point:

Edward Thomas wrote poetry throughout World War I.

This cannot be true when he was killed long before the war ended. The other problem is that people routinely use a phrase such as throughout the entire even though that is redundant: throughout already implies entire, unless modified by some other phrase:

The two teams stayed close throughout.

This implies that the final score was close. If it wasn’t, then you would have to write something like

The two teams stayed close throughout the first half, but in the third quarter the Patriots pulled away.

This word is not a fancy way of saying different. It is an absolute (like opposite) and literally means that whatever is being described is the only one of its kind. Therefore, you cannot sensibly modify this adjective with adverbs such as very, extremely, fairly, rather, or quite. This is one of the most commonly misused words in the English language.

These are almost mutually exclusive means of measurement. The rule is that you should use number when something can be counted, and amount when it can only be measured in some other way. Thus, someone can drink an amount of (or a little, less, much, too much) beer, but the same person drinks a number of (or few, fewer, many, several, too many) beers. Another way to think of it is that you use number when the noun can become plural and amount when it cannot:

Bobby is sick; he ate too many hot dogs and too much chicken.

Consider how wrong the alternative sounds:

Bobby is sick; he ate too much hot dog and too many chickens.

On the other hand,

How many chickens do you see in the yard?

sounds fine because live chickens can be counted. Only more works for both amount and number.

Other examples:

“I have a huge amount of homework to do this weekend.”
“Yeah? Well I have fewer classes than you do, but I still have several papers to write.”
“True, but I have less time after work”
“What do you mean? We work the same number of hours.”  
“Yes, but my shift is later, so you have more time after work to write, more hours before you are tired.”

And yes, the signs at most grocery stores — “X items or less” — are wrong; they should say “X items or fewer.” Locally, Wegmans is the only store that uses the correct phrase: score one for grammar! (On the other hand, the store does not use an an apostrophe in its name, even though the family that founded it is named Wegman, so one step forward, one step back.)

Common Punctuation Problems

If a comma is needed with a conjunction, it belongs before the conjunction, not afterwards. The natural place to pause is before the conjunction. Read the next two lines aloud and see which sounds more natural:

Wrong: The blood-dimmed tide is loosed and, everywhere the ceremony of innocence is drowned.
Right: The blood-dimmed tide is loosed, and everywhere the ceremony of innocence is drowned.

This problem also occurs when people confuse adverbs and conjunctions, most commonly (but not always) at the beginning of a sentence. When starting a sentence with an adverb such as However or Thus, a comma is often required. But a comma is incorrect after coordinating conjunctions (the problem happens especially with but and yet) and subordinating conjunction (especially although):

Wrong: But, she never returned to her hometown.
Right: But she never returned to her hometown.

Wrong: Although, Machiavelli has a reputation for being ruthless and amoral, in reality he was a conscientious public servant.
Right: Although Machiavelli has a reputation for being ruthless and amoral, in reality he was a conscientious public servant.

The only time you should put a comma after a conjunction is when a phrase that interrupts the main phrase follows:

Right: Yet, she reflected, the opportunity might not come again.

When using multiple commas in a series, the Oxford comma — the comma before the conjunction and last item in a series — is your friend. Although not an error per se, omitting it can be confusing. Use a comma and conjunction (instead of just a conjunction) after the next-to-last item in any series of three or more items, especially if the series contains phrases, not just single words:

Confusing: The actors in the play included a trained Shakespearian from London who had never played a role more important than “Second Lord,” an alcoholic grandmother who hadn’t been on stage in a dozen years, a former Academy Award nominee who now couldn’t remember more than two lines in a row without a cue card, an aging soap opera heartthrob with an insatiable hunger for cocaine and various ingenue refugees from B-level horror flicks — all of whom couldn’t stand one another.

Does the aging soap opera heartthrob hunger for cocaine and ingenues? Or are the ingenues performing in the play? We have no way of knowing here.

Not confusing: The actors in the play included a trained Shakespearian from London who had never played a role more important than “Second Lord,” an alcoholic grandmother who hadn’t been on stage in a dozen years, a former Academy Award nominee who now couldn’t remember more than two lines in a row without a cue card, an aging soap opera heartthrob with an insatiable hunger for cocaine, and various ingenue refugees from B-level horror flicks — all of whom couldn’t stand one another.

In this case, the ingenues are clearly in the play.

Do not confuse dashes and hyphens; they are not the same punctuation mark, and they have completely different functions. A hyphen, which looks like this - connects what is on either side of it; a dash, which looks like this — separates what comes before it from what comes after it. Use a hyphen for a hyphenated word, or (rarely necessary nowadays) to connect the first part of a word on one line to the last part on another. Use a dash to interrupt a sentence, similar to a colon or a parenthesis, except that a dash gives more emphasis, whereas parentheses suggest that what is inside them can be ignored. You can find a dash on the insert menu in most word processing programs; otherwise you may substitute two hyphens -- like that (some word processing programs will automatically change this to a dash anyway). In any case, to make both easier to recognize, it helps to put spaces around dashes but not around single hyphens.

Wrong: World War I ace Manfred von Richthofen, known as the Red Baron, and his brother Lothar were members of a well—known aristocratic Prussian family-surprisingly their cousin Frieda was married to the famous British writer D. H. Lawrence.
Right: World War I ace Manfred von Richthofen, known as the Red Baron, and his brother Lothar were members of a well-known aristocratic Prussian family — surprisingly, their cousin Frieda was married to the famous British writer D. H. Lawrence.
Also permissible: World War I ace Manfred von Richthofen, known as the Red Baron, and his brother Lothar were members of a well-known aristocratic Prussian family -- surprisingly, their cousin Frieda was married to the famous British writer D. H. Lawrence.

You should generally avoid exclamation points (except in dialogue or when stating a command). Let your words speak for themselves. Yelling does not make you more persuasive.

Wrong:  In effect, Tom Buchanan is a vicious and unrepentant murderer!
Right:  In effect, Tom Buchanan is a vicious and unrepentant murderer.

Quotation marks are for quotations and the titles of some kinds of artistic works. You should not use them for emphasis or to indicate verbal irony (sarcasm). In academic work, especially, you should not use quotation marks capriciously. When a reader sees them in your writing, he or she will wonder why you are quoting and quite possibly look for a citation of some kind. The term for using quotation marks when you are not actually quoting someone is air-quoting. Ever talk to people who air-quotes in conversation? They will say something like

So I was talking to my “boss,” and she says I have to show more “initiative” at work if I want to “get ahead,” as if I care what some “Assistant Regional Manager” thinks.

Not only will the person’s voice go up a few tones on the words with quotation marks around them, but sometimes these people actually raise their hands to shoulder level and make little hooking motions with their index and middle fingers in order to indicate the air-quotes. It’s an awful habit, so why would you want to do it in your writing?

Common Stylistic Errors (some are incredibly annoying, some just best avoided)

Wrong:  Now, you would think he’d have been embarrassed. Well, he wasn’t. See, he didn’t care.
Right:  You would think he’d have been embarrassed. He wasn’t. He didn’t care.

These words are different from transition words such as however, nonetheless, and moreover. Those words help the reader understand the relationship between the information presented in the prior sentence and the information that follows; conversational words merely take up space without providing any real information.

Most people realize that saying not twice in a row is poor phrasing. But the same is true of any two words that serve a negative function:

Wrong: I didn’t say nothing after that.
Right: I didn’t say anything after that.
Or: I said nothing after that.

Wrong: She has never not loved to draw.
Right: She has always loved to draw.

The problem is that so indicates a causal relationship with the phrase or clause that came immediately prior, so making the so clause a separate sentence is counter-intuitive. Usually you are better off combining (and often shortening) the sentences or using thus or therefore:

Wrong: Hamlet believes Rosencrantz and Guildenstern have betrayed him. So he has them killed.
Right: Hamlet believes Rosencrantz and Guildenstern have betrayed him, so he has them killed.
Also Right: Hamlet believes Rosencrantz and Guildenstern have betrayed him. Therefore, he has them killed.
Better: Hamlet has Rosencrantz and Guildenstern killed because he believes they have betrayed him.
Also better: Believing Rosencrantz and Guildenstern have betrayed him, Hamlet has them killed.

One exception to this rule is when so introduces a dependent clause:

“So that Sir Cecil would not show up for his speech drunk, I assigned my assistant Penelope to watch over him all day.”

In that case, the sentence’s normal syntax (I assigned my assistant Penelope to watch over him all day so that Sir Cecil would not show up for his speech drunk) has simply been inverted, which is perfectly acceptable. But unless you are sure of what you are doing, just avoid the problem.

So and such express incomplete concepts by themselves; you need to follow either of them with a phrase elaborating on and completing the idea:

Wrong: He is so ugly.
Right: He is so ugly that blind people who commit crimes should have to feel his face as a punishment.
Wrong: She was such a fan of James Dean.
Right: She was such a fan of James Dean she bought all three of his films on Blu-Ray even though she already owned them all on DVD.

Whenever people use the word fact, the chance is about 80% that what follows is just an opinion:

Wrong: The fact is Joe Gibbs was the best football coach of the Super Bowl era.
Right: Because he won three championships with three different quarterbacks — none of whom has even a remote chance of being inducted into the Hall of Fame — Joe Gibbs was the best football coach of the Super Bowl era.

This is not actually a grammatical error, but using who or whom when appropriate has two advantages: it creates variety because that is one of the most common words in the language, and it comes across as both more precise and more respectful.

Wrong: The pianist that played was over eighty years old.
Right: The pianist who played was over eighty years old.

In contemporary writing, that introduces a restrictive phrase, meaning a phrase essential for the meaning of the sentence. On the other hand, which can introduce either a restrictive phrase or a nonrestrictive phrase, meaning a phrase that provides additional but not required information. A restrictive phrase is never set off with commas; a nonrestrictive phrase is set off with commas (or sometimes parentheses or dashes are used instead):

Wrong: She recited the poem, that she had memorized fifty years earlier, flawlessly. Seeing all those people cheering when she was done was something which I will never forget.
Right: She recited the poem, which she had memorized fifty years earlier, flawlessly. Seeing all those people cheering when she was done was something that I will never forget.

Note that sometimes context makes the difference. Both of the following phrases could be correct:

The house that burned down was over two-hundred years old.
The house, which burned down, was over two-hundred years old

The difference is that the first sentence would be appropriate if it were in the context of a story or conversation about a fire; in that case, the house burning down is crucial information. The second sentence would be appropriate if the house burning down is merely incidental information, for example if the story or conversation is about the early history of a particular house.

We call this parallelism or parallel structure. Parallelism is both pleasing to the ear and easy to remember. Many famous lines in speeches employ parallel structure for that reason. In contrast, poor parallel structure is a syntactical error that often sounds worse than some grammatical errors.

Wrong: I like to swim, to do yoga, and playing basketball.
Right: I like swimming, doing yoga, and playing basketball.

If you cannot gracefully make one of the series into the same form as the rest, you can get around the problem by putting the last item in the series in a separate phrase:

Wrong: Richard Francis Burton became famous for his explorations, his many books, and shocking the Victorian public with his admiration for supposedly more primitive cultures.
Right: Richard Francis Burton became famous for his explorations and his many books, and infamous for shocking the Victorian public with his admiration for supposedly more primitive cultures.