Which of these sentences uses grammar correctly?
joan is the person who hired me.
joan is the person that hired me.
joan is the person whom hired me.

The sentence that uses grammar correctly is: Joan is the person who hired me. The key is to use the right pronoun.  

A pronoun is used to substitute a noun. In order for it to substitute, it must have a clear antecedent. Personal pronouns are used to substitute nouns with ownership. There are three persons point of view.1st person is when the subject is the one who is speaking (e.g. I, me, my, mine, we, us, our, ours). 2nd person is when the subject is the one being spoken to (you, your, yours). 3rd person is when the subject is the one spoken about (he, him, his, she, her, hers, it, its, they, their, theirs).