5. Interrogative (asking questions) and Relative (used to refer to a noun in the main clause) Pronouns. All are both singular and plural.

Who and Whom; Whoever and Whomever should be used:

a. Who is going? (interrogative) To whom shall I direct the question? (interrogative)
    She found herself the only one there.
b. She is the one who is going. (relative) Mary, whom I have not met is going. (relative)
c. Who and whoever is the nominative (subject) form
d. Whom and whomever are the objective form.

6. Who, Which, and That

Who and that are used when referring to persons.:

a. Select "who" when the individuality of a group is meant and that when a class, species, or type is meant.
    She is the only one who can speak Chinese. He is the kind that tells the truth
b. Which and that are used to refer to places, things, and animals
    Which dog will you choose? That cat needs some water.