10. Possessive forms of Personal Pronouns

The possessive form of the personal pronouns and the possessive form of the pronoun who are never written with an apostrophe. These pronouns have a special form to show possession and do not require an apostrophe. Do not place an apostrophe either before or after the s in any of these words. The correct forms to use in order to show possession are the following:

my mine yours his hers its ours theirs whose

The word it's is a contraction of it is. It is not a form of the pronoun, and should never be used to show possession. The word who's is a contraction of who is or who has, and should not be confused with the possessive form whose.

It's on your desk. (It is on your desk)
Who's helping me today? (Who is helping me today?)
Who's finished their lesson? (Who has finished their lesson?)