We use 'object pronouns' instead of object nouns.
We use 'subject pronouns' instead of subject nouns.

(ex) You might go to the park with they and their friends (X)
(-> You might go to the park with them and their friends)
(ex) I and she have to read the books until this December.

We use possessive adjectives and possessive pronouns.

(ex) When we had a dinner, I paid mine, my friend paid his.
(ex) This computer is yours.

All pronouns should agree with the nouns it refer to.
ex : After prepositions, you should use object pronouns.

(ex) If you have a problem, please tell me about them.
(ex) Each student needs to have his own computer to improve his skill.