What is a pronoun? It’s simply a word that takes the place of a noun.
Pronoun cases: Subject, Object, Possessive, Reflexive
1) Subject: I, you, he, she, it, we, they
Use subject pronouns when…
(a) the pronoun is the subject of the sentence:
I am the boss.
She is my friend.
We are out of time.
(b) the pronoun renames the subject*:
It was she.
This is he speaking.
It was they who chose not to attend.
*follows “to be” verbs: is, are, was, were, am, will be
2) Object: Me, you, him, her, it, us, them
Use object pronouns everywhere else.
Mary walked with us.
When can you visit me?
I met him yesterday.
Here’s where it gets a little tricky… How do you decide whether the Subject or Object pronoun should follow the words than or as? See if you can choose correctly by completing the sentence:
John is as punctual as she/her.
Completing the sentence, it reads: John is as punctual as she (is).
Everyone is faster than I/me.
Completing the sentence, it reads: Everyone is faster than I (am).
3) Possessive: mine, yours, his, hers, its, ours, theirs
Use possessive pronouns to show ownership – they NEVER take an apostrophe.
Did you pick up her dry cleaning?
The car is ours tonight.
Have they found their dog yet?
Note: Its is a possessive pronoun. It’s is a contraction meaning “it is” or “it has”. Remember: Possessive pronouns never take an apostrophe.
4) Reflexive: myself, yourself, himself, herself, itself, ourselves, yourselves, themselves
Reflexive pronouns are always used as objects, when the subject and the object are the same and to emphasize the subject.
I injured myself.
My father can drive himself to the doctor.
She talks to herself when under stress.
We can do the job ourselves.
Mary and myself have the time to walk the dog.
Correct: Mary and I have the time to walk the dog.
Bill gave John and yourself his change of address.
Correct: Bill gave John and you his change of address.
Note: The trick to using reflexive pronouns correctly is to remember that they’re always used as objects and must refer back to another word in the sentence.
Source: The Blue Book of Grammar and Punctuation by Jane Strauss
I highly recommend this easy-to-use grammar guide. Besides great instruction, it comes with lots of quizzes to give you practice.