Rule one: If possessive noun is singular, then you need to add apostrophe plus “s”. Rule two: If possessive noun is not ending with the letter “s”, then you always need to add apostrophe plus “s”. Rule three: If possessive noun ends with the “s” and it is plural, what you only need to do is to add apostrophe.

Apostrophes should not be used with possessive pronouns because possessive pronouns already show possession—they don't need an apostrophe. His, her, its, my, yours, ours are all possessive pronouns. However, indefinite pronouns, such as one, anyone, other, no one, and anybody, can be made possessive.

To show possession using an apostrophe, add ’s for individuals (“Smith’s car”) and just the apostrophe after the s for plurals (“the Smiths’ car,” “the Martinezes’ dog”). By convention, names from classical mythology and the Bible ending in s show possession with the apostrophe only (“Jesus’ teachings”).