Object

Like the subject of a clause, an object must be a noun.

An object can be the object of a verb:
I saw the dog.

An object can be the object of a [preposition]:
I opened the door for the dog.

For most personal pronouns, the subject form is different from the object form.
* me
* her
* him
* us
* them
The second person pronoun used to have a difference between subject and [object] and a difference between singular and plural, but we lost those differences about two hundred years ago.
* thee
* you