Frequently Asked Questions About Grammar and Style

• When do I use “that” instead of “which”?
• When do you use a semicolon?
• How do I use apostrophes?
• What’s a dangling modifier?

When do I use “that” instead of “which”?

Your choice of “that” or “which” at the beginning of a clause depends upon whether the information in the clause is essential to the sentence. The clause is "essential" not because it contains information you think is important, but if dropping it would fundamentally alter the meaning of the sentence.

“That” or “which” — no comma

Take for example, “You should not ride motorcycles that are dangerous.”

In this sentence, what comes after the “that” is essential, because it restricts which motorcycles (only the dangerous ones) should be avoided. In this case, the "that"-clause is a "restrictive modifier."

In cases where the modifying information is essential, or restrictive, you may use either “that” or “which,” though some people — and some disciplines — prefer "that" with restrictive clauses. If you do use "which," make sure not to use a comma before it; if you do, the sentence means something quite different, as we'll see below!

“Which” only — use a comma

In the case of “non-restrictive modifiers,” the information — however interesting or important it may be — is not grammatically essential. Use “which” and separate the clause from the main sentence with commas:

“You should not ride motorcycles, which are dangerous.”

Here, the "which"-clause does not restrict the group of motorcycles you should avoid — according to this sentence, all motorcycles are dangerous and should not be ridden. This non-restrictive modifier is not essential to the sentence; if you just said, "You should not ride motorcycles," you would convey the same meaning to your listener. What a difference a comma can make!

Another example: "I bought my pick-up truck, which is red, in 2001."

Here the clause "which is red" is inessential, so it must begin with "which" and be surrounded with commas.

For more information, see our Writer's Handbook page on Punctuating Restrictive and Non-Restrictive Modifiers.

How do I use apostrophes?

Apostrophes (’) are used for two main reasons: to signify possession or to join two separate words as a contraction. Do not use an apostrophe to make a word plural.

1) Apostrophes to signify possession:

Singular Possessive: add an apostrophe + s to make a noun possessive

Miss Parker’s high heels
[the high heels belonging to Miss Parker]

the professor’s lecture [the lecture of the professor]

According to the Chicago Manual of Style, when the singular noun taking the possessive form already ends in an “s,” the same rule generally applies:

Brooks’s essay
Texas’s flag
Kirk Douglas’s films

However, this is not the case in journalism and some other fields.

Plural Possessive: add an apostrophe alone (after the “s” that is already there)

the Coen Brothers’ movies
[the movies of the Coen Brothers]

the rockets’ red glare
[the red glare of the rockets]

Possessive Pronouns: do not use apostrophes

your [belonging to you]
its [belonging to it]
their [belonging to them]

2) Apostrophes to form contractions:

In this case, part of a word is replaced by an apostrophe and joined to the word which precedes it.

you’re [you + are]
it’s [it + is]
they’re [they + are]

Be careful not to confuse contractions and possessive pronouns.

It’s easy to confuse words that sound alike, like "its" and "it's." Double-check your work to make sure that you haven’t used a contraction where you needed a possessive pronoun, or vice versa.

If you've gotten the hang of apostrophes, take our self-test!

For examples of other common punctuation and style errors, take a look at our Editing Checklist.

What’s a dangling modifier? (And what’s a modifier, for that matter?)

A modifier is a phrase or word meant to describe or explain part of a sentence. In Example 1a, the phrase before the comma modifies the subject of the main sentence, which is "I":

Example 1a: Driving down the interstate, I saw the dead dog.

The modifer “driving down the interstate”clearly describes the action of “I.”

A dangling or misplaced modifier is not clearly connected to its object, making the sentence unclear or illogical:

Example 1b: I saw the dead dog driving down the interstate.

In Example 1b, the modifier appears to describe “the dead dog,” an unlikely candidate for a driver’s license. The problem with dangling modifiers is that they confuse your reader. To get an idea of how embarrassing this kind of mistake can be, take a look at our list of the best dangling and misplaced modifiers of all time.

Dangling modifiers aren’t always that obvious, however, which is why they are so common. It’s especially easy to read right over them when you already know or can easily guess what the modifier refers to. Here’s a trickier example:

Example 2: A sinister genius, Agent Starling had to be on her guard against Hannibal Lecter at all times.

In Example 2, the modifier is supposed to describe "Hannibal Lecter," but instead it appears to modify "Agent Starling," the subject of the sentence. If your reader has seen The Silence of the Lambs, s/he could probably figure out that “a sinister genius” refers to Lecter, not Starling. If your reader hasn’t seen the movie, though, s/he might think that Starling is the sinister genius, since that's what the sentence actually says. That would be quite confusing.

To avoid this error, place modifiers near the words they describe; be sure the modified words actually appear in the sentence.

For examples of other common punctuation and style errors, take a look at our Editing Checklist.