1. Use an apostrophe in possessive forms:

2. Use an apostrophe in contracted forms (the apostrophe shows that letters have been left out):

Certain words are sometimes written with an apostrophe (to show that they are really a shortened form of the original, longer word):

Some people use an apostrophe when the first two figures of a year are left out:

3. You can use an apostrophe to show the plural of letters and numbers:

You can use an apostrophe to show a plural form for words that are not normally plural:

Possessive pronouns or determiners (except one's) do not use apostrophes. Do not confuse them with contractions. The following are typical mistakes:

Wrong Right
It's value is Its value is
It's going to rain
Who's are these? Whose are these?
Who's coming?
These are your's These are yours
exception → One's self-esteem