Personal pronouns represent specific people or things. We use them depending on:
number: singular (e.g: I) or plural (e.g: we)
person: 1st person (e.g: I), 2nd person (e.g: you) or 3rd person (e.g: he)
gender: male (e.g: he), female (e.g: she) or neuter (e.g: it)
case: subject (e.g: we) or object (e.g: us)

Examples (in each pair, the first sentence shows a subject pronoun, the second an object pronoun):
1. I like coffee. / John helped me.
2. Do you like coffee? / John loves you.
3. He runs fast. / Did Ram beat him?
4. She is clever. / Does Mary know her?
4. It doesn't work. / Can the man fix it?
5. We went home. / Anthony drove us.
6. Do you need a table for three? / Did John and Mary beat you at doubles?
7. They played doubles. / John and Mary beat them.