Below are rules to show you how to use possessive pronouns.
Apostrophes are not added to possessive pronouns because they are already in their possessive form.
Incorrect: her's or hers'
correct: hers
Possessive pronouns are used to show who or what owns something.
Remember that a possessive pronoun can replace a possessive adjective and a noun to avoid repeating information that has already been made clear.
Theirs can replace their car.
Hers can replace her brush.
A possessive adjective appears in front of a noun as in:
That is their car. (possessive adjective their + noun car)
A possesive pronoun replaces a possessive adjective and a noun.
That is theirs. (possessive pronoun)
Language Arts: Possessive Pronouns