The pronouns who and whoever are in the subjective case, and whom and whomever are in the objective case. Within each case, the pronouns do not change form for singular or plural, and they do not change form for fist, second, or third person.

Knowing how to use who, whoever, whom, and whomever in dependent clauses
        A dependent clause contains a subject and predicate and starts with a word that makes the clause unable to stand alone
as a sentence. Pronouns such as who, whoever, whom, or whomever start many dependent clauses.
        To determine what pronoun case is correct in a dependent clause, it is not necessary to determine whether the entire clause is functioning as a subject or an object in the sentence.

Cases of Relative and Interrogative Pronouns

Subjective	Objective	Possessive
who	whom	whose
whoever	whomever

        If you want to check your use of who and whom, try the drop test. Temporarily drop everything in the sentence up to the pronoun in question, and then make substitutions. Remember that he, she , they, who, and whoever (the -m forms and her) are objects. Here is how the method works for the subjective case.

Test for Who/Whom in the Subjective Case

Example I wondered (who, whom) would vote. Step 1 - Omit "I wondered." Step 2 - Test the sentence with he and him: "He would vote" or "Him would vote." Step 3 - Answer: "He would vote." Step 4 -  Therefore, because he is subjective, who which is also the subjective is correct: "I wondered who would vote."

        This four-step drop test works also for whoever.
        Voter registration drives attempt to enroll whoever is eligible to vote. ["He (not him) is eligible to vote" proves that the subjective case of whoever is needed.]

Test for Who/Whom in the Objective Case

Example Volunteers go to senior citizen centers hoping to enroll people (who. whom) others have ignored. Step 1 - Omit "Volunteers...people." Step 2 - Test the sentence with they and them:  "Others have ignored they" or "Others have ignored them." Step 3 - Answer:  "Others have ignored them." Step 4 - Therefore, because them is objective, whom, which is also objective, is correct:  "Volunteers go to senior citizen                centers hoping to enroll people whom others have ognored.

        This four-step drop test works also for whomever:
        The senior citizens can vote for whomever they wish. ["The senior citizens can vote for him" proves that the objective case of whomever is needed.]