Here's why:
* Objective Case: Used for pronouns that are the object of a verb or preposition. Examples: "I saw her." "The book is for her."
* Subjective Case: Used for pronouns that are the subject of a verb. Example: "She went to the store."
* Possessive Case: Shows ownership. Example: "That is her book."
Let me know if you'd like more examples or have any other pronoun questions!