Who, whoever, whom, and whomever are pronouns— words that take the place of a noun in a sentence.
Who and whoever are subjective-case nouns, which simply means that they are used as substitutes for subjects in a sentence.
He tells that story to whoever will listen. Whoever is the subject of will listen.
Whom and whomever are objective-case nouns, which means that they take the place of an object in a sentence. Because of this, whom and whomever are often (but not always) found after a preposition (to, of, with, about, for, from, etc.).
The board will probably approve of whomever we select. Whomever is the direct object, receiving the action “approve.”
Who/whoever and whom/whomever are used in two main functions:
Relative or Subordinate Clauses
In these sentences, who/whoever or whom/whomever introduce subordinate (or dependent) clauses – phrases that can’t exist without the rest of the sentence.
Salvador Dalí was an artist who took great delight in shocking his contemporaries.
“who took great delight in shocking his contemporaries” is the dependent clause – if you use it without the first part of the sentence, it will be a fragment: an incomplete sentence.
You will meet with our senior engineers, whom you will meet later.
“whom you will meet later” is the dependent clause – if you use it without the first part of the sentence, it will be a fragment, an incomplete sentence.
Interrogative Pronouns (in Questions)
When used as an interrogative pronoun, who/whom usually begins a sentence.
Who left the window open? OR Whom is the teacher speaking to now?
Hacker, Diana. A Pocket Style Manual. Boston: Bedford/St. Martin's, 2000. Perrin, Robert. The Beacon Handbook and Desk Reference. Boston: Houghton Mifflin, 2003.