PRONOUN

Subjective Pronoun                        : I       - We   - You  - They   - He  - She - It
Objective Pronoun                         : Me   - Us    -  You  - Them  - Him - Her - It
Possessive Pronoun Before Noun: My   - Our  -  Your - Their  - His  - Her - Its
Possessive Pronoun After Noun   : Mine - Ours - Yours - Theirs - His - Hers - Its

Examples:
We are the best student.
He looked at me.
Your friend is an actor.
That house is theirs.

Each Pronoun can't be replaced each other, such as:
Us are the best student.  --> this sentence is totally Wrong.