One of the biggest mistakes in written English is the improper use of apostrophes.

Consider the following examples:

In the wrong versions of these example sentences, an apostrophe is used to indicate possession. When you're using pronouns, you don't use an apostrophe because in the English language we have what are known as possessive pronouns.

Many people become confused about these constructions. You should also be on the lookout for the tendency to make words plural by using an apostrophe with an s at the end. An apostrophe s is only used to indicate possession or contraction, not to make a noun plural.

Whose dog is that?

That's Sara's dog.

The apostrophe s following that indicates a contraction. If the sentence were spelled out in full, it would read “That is Sara's dog.” The apostrophe s takes the place of the is. The apostrophe s following Sara, however, is a possessive adjective. It could be stated differently too:

Whose dog is that?

That's the dog of Sara.

Yes, the sentence sounds terrible that way, but it serves to illustrate another way you can indicate possession. (A better way would be: That dog belongs to Sara.) When you're using apostrophes, then, verify that you're using them in the right circumstances by seeing if you could construct the sentence any other way. If the word with the apostrophe is a contraction, it can be stretched out to its full version. If it's a possessive apostrophe, you should be able to reconstruct the sentence using the word of to indicate possession. If you can't do either of those, then it's probably used with a plural noun that should not take an apostrophe in the first place.

It also sometimes helps to wiggle the words in your sentence the other way: If you're confused about whether to use its or it's, remember that it's is the contraction of it is; its therefore is the possessive pronoun. Likewise, whose and who's also often cause confusion. Who's is the contraction of who is (as in “Who's coming to dinner?”) and whose is the possessive pronoun (as in “At whose house will we be meeting for lunch?”).