Just in time for Halloween, and before the owl swoops down on my black cat, I’m gonna tell you the difference between who and whom. It’s tough to decide whether to use who or whom in a sentence (or whoever/whomever). The general rule is that who and whoever are used for subjects and subject complements.  Whom and whomever are used for objects. Here are some examples:

The prize goes to the runner who collects the most  points.

In the subordinate clause, who is the subject of the clause who collects the most points. Since a
subject for the verb collects is needed, the correct choice is who.

He tells that story to whoever will listen.

In the subordinate clause, whoever is the subject of the verb will listen. Since the verb must
have a subject, whoever is the correct choice.

Subject complements occur with linking verbs such as be, am, is ,are, was, were, being, and been.

The receptionist knows who you are.

In this example, the sentence uses are which is a linking verb. After a linking verb, you do not use an object, instead you use a subject complement. The correct choice, therefore, is who.

Inserted expressions such as they know, I think, and she says should be ignored in determining the case of a relative pronoun. Here is an example.

All of the show-offs in school want to take on a big guy who they know will not hurt them.

In the clause who (they know) will not hurt them a subject is needed for the verb will not hurt.
So, who is the correct choice for the subject of that verb.

In questions, you should use who and whoever for subject and whom and whomever for objects.

Who is responsible for this mess?
Whom did the committee select?

In the first example, there is no subject for the verb is, so you must use who. In the second
example, committee is the subject of the verb select. So, the objective pronoun whom is the
correct choice for this question.

Use that to refer to things, not people.
Bill has the newspaper that I read. Al hit the ball that went over the fence.