Here's why:
* Subject pronouns are used when the pronoun is the subject of the sentence (the one doing 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 (the one receiving the action). Examples: me, you, him, her, it, us, them.
Example:
* Subject: They went to the store.
* Object: I saw them at the store.
In the second example, "them" is the object of the verb "saw."