Here's why:
* Subject pronouns are used when the pronoun is the subject of a verb (performing the action). Examples: I, you, he, she, it, we, they.
* Object pronouns are used when the pronoun is the object of a verb or preposition (receiving the action). Examples: me, you, him, her, it, us, them.
Example:
* Subject: They went to the store. ("They" is the subject performing the action "went")
* Object: I saw them at the store. ("Them" is the object of the verb "saw")