The case of a noun/pronoun/adjective shows its job in the sentence.
This case is used for the subject of a sentence or clause. It names the person or thing the sentence is about. With an active verb, it refers to the person who does the action:
puella librum legit. The girl reads the book.
With a passive verb, the subject is the person or thing on the receiving end of the action:
liber a puella legitur. The book is read by the girl.