Here's why:
* Subject Case: Used when the pronoun is the subject of a verb (doing the action).
* Example: You are going to the store. (You is the subject of "are going")
* Objective Case: Used when the pronoun is the object of a verb or preposition (receiving the action).
* Example: I saw you at the store. (You is the object of the verb "saw")
* Example: I gave the book to it. (It is the object of the preposition "to")
Let me know if you have any other grammar questions!