The subjective, or nominative, pronouns are I, you (singular), he/she/it, we, you (plural), they and who. The subjective pronouns are the subjects of the sentence.
I have a big chocolate bar.
You have some ice cream.
He has a cake.
We could have a party.
They could come, too.
Who should be invited?
The objective, or dative, case pronouns are me, you (singular), him/her, us, you (plural), them and whom. We use the objective case when something is being done to (or given to, etc.) someone.
Give the chocolate to me, please?
Why should I give it to you?
You could give it to him, instead.
Please share it with all of us?
Do we have to share it with them?
The possessive pronouns are mine, my, your, yours, his, hers, its, their, theirs, our and ours. The possessive pronouns shows that something (or someone) belongs to someone (or something).
That’s my shirt.
That shirt is mine.
The house is theirs.
It’s their house.
The dog is scratching its ear.
It’s scratching its ear.
Notice that the first pronoun is part of a contraction, it’s, which is short for it is. The possessive pronoun doesn’t have an apostrophe.