Here's why:
* Subject pronouns are used when the pronoun is the subject of a sentence, meaning it performs the action of the verb.
* Examples: I, you, he, she, it, we, they
* Object pronouns are used when the pronoun is the object of a verb or preposition, meaning it receives the action of the verb or is the focus of the preposition.
* Examples: me, you, him, her, it, us, them
* Possessive pronouns show ownership.
* Examples: my, your, his, her, its, our, their
Therefore, only subject pronouns can be used as the subject of a sentence.
Here's an example:
* Correct: He went to the store. (Subject pronoun)
* Incorrect: Him went to the store. (Object pronoun)