You should use an apostrophe to form the possessive case of a noun or to show that you have left out letters in a contraction. Note that you should not generally use contractions in formal, academic writing.
- The convertible's engine has finally died. (The noun "convertible's" is in the possessive case)
- I haven't seen my roommate for two weeks. (The verb "haven't" is a contraction of "have not")
To form the possessive of a plural noun ending in "s," simply place an apostrophe after the "s."
- He has his three sons' futures in mind.
- In many suburbs, the houses' designs are too much alike.
Possessive pronouns -- for example, "hers," "yours," and "theirs" -- do not take apostrophes. This is the case for the possessive pronoun "its" as well: when you write "it's" with an apostrophe, you are writing a contraction for "it is."
- The spaceship landed hard, damaging its radar receiver. ("its" is the possessive pronoun)
- It's your mother on the phone. ("it's" is the contraction of "it is")
Written by Frances Peck