Personal pronouns change case according to how they are used in a sentence. Pronouns functioning as subjects (or subject complements) are in the subjective case; those functioning as objects are in the objective case; and those functioning as possessives are in the possessive case.
Pronoun case
Exercises:
Pronoun case (such as I vs. me) 1
Pronoun case (such as I vs. me) 2
Pronoun case (such as I vs. me) 3
Pronoun case: review 1
Pronoun case: review 2