Here is the simple rule I use: in English, pronouns never use an apostrophe for the possessive. He --> his She --> her They --> their We --> our I --> my

Here is the simple rule I use: in English, pronouns never use an apostrophe for the possessive. He --> his She --> her They --> their We --> our I --> my You --> your It --> its (Since someone will probably point this out, this is only true of the common pronouns. There are unusual pronouns that have apostrophes in their plurals. Eg: one --> one's. But nevertheless, the rule worked for me.)