Use these with nouns or
noun phrases:

What's my / your / his / her / its /
our / their problem?

Jill doesn't have my / your / his /
her / our / their home phone number.

Use these alone. Don't use them
with nouns or noun phrases:

Everyone has problems. What's
mine / yours / his / hers / ours / theirs?

Joe has everyone's home phone
numbers except mine / yours /
hers / ours / theirs.

Important: Adjectives are not plural in
English, so possessive adjectives have
only one form:

That's my / your / his / her / our / their key.

Those are my / your / his / her / our / their keys.

Important: Do not use possessive pronouns
with nouns or noun phrases:

< FONT COLOR="#ff0000" SIZE="+1">wrong: Those are *theirs books.

wrong: Have you met *mine brother?

The words its and it's are pronounced the
same, but they have different meanings
and grammar:

its = possessive adjective

it's = contraction for it is or it has

Do not use it's for a possessive pronoun:

wrong: What are *it's main advantages?

wrong: Read the instruction manual.
*It's directions are easy to understand.