The apostrophe is used to replace missing letters in contractions, to indicate the possessive form of certain nouns, and to form the plural of some words in very specific situations.
1. Use an apostrophe in a contraction to indicate missing letters in words or missing numbers in a year.
|can't (can not)||he'd (he would)|
|I'm (I am)||it's (it is or it has)|
|you're (you are)||'99 (1999)|
|haven't (have not)||'03 (2003)|
|there's (there is)||o'clock (of the clock)|
2. Use an apostrophe in the possessive form of a noun to indicate ownership. To show
ownership, add apostrophe + s to the end of a word, with one exception: To show ownership with a plural noun already ending in s add only the apostrophe.
A boy's shirt is under the chair.
The child's toy was on the shelf.
Lois's book is fascinating.
The bus's door was still open.
The men's hats were purchased at our store.
She wanted to be the people's princess.
The boys' uniforms were in the locker room.
She found the birds' nests under the tree.
3. Avoid common mistakes using the apostrophe.
The fruit stands were selling apples, pears, and walnuts.
I'm sure you realize it's a holiday. (It's is a contraction for it is.)
It's been such a long time. (It's is a contraction for it has.)
The house lost its roof in the storm. (The possessive pronoun its already indicates
ownership by definition and therefore needs no apostrophe.)
The word you're is a contraction and should be used only in place of you are. The word your is a possessive pronoun and never takes an apostrophe.
I presume you're planning to bring your coat on the hike. (I presume you are planning to bring the coat that belongs to you on the hike.)
Who's on first? (Who's is a contraction for who is.)
Who's been turning off the lights? (Who's is a contraction for who has.)
Whose book is on the table? (Whose is a possessive pronoun that already indicates ownership by definition and therefore needs no apostrophe.)
4. Additional rules apply to the use of apostrophes in certain specific situations.
|maybe's||8's||I.D.'s||A's and B's|
However, MLA format differs somewhat by recommending no apostrophe for the plural form of numbers and abbreviations.
|1980s||VCRs||the ‘70s||7s and 8s|
Her brother-in-law's car was an antique.
Maria and John's wedding lasted all day.
Maria's and John's expectations of marriage were very different.