Who is that masked man? (“Who” / subject) 

    The men, four of whom are ill, were indicted for fraud. (“whom” / object) 

    When “who” is not the main subject of the sentence, however, many people become confused. They tinker and change who to “whom.” 

    It was Thomas Jefferson, I think, who was the third president of the United States.

    Notice that “who,” not “whom,” is still the correct form as the subject of the clause that follows. The proper name, Thomas Jefferson, could be substituted for “who” to make a perfectly good sentence: 

    Thomas Jefferson was the third president of the United States. 

    As a ready check in such sentences, simply substitute the personal pronoun “he/him” or “she/her” for “who/whom.” If he or she would be the correct form, the proper choice is who.” If  “him” or “her” would be correct, use “whom.” 

    This technique of substituting a personal pronoun for the relative pronoun works nicely whenever you have difficulty deciding whether to use “who” or “whom,” assuming that you have no difficulty using the proper form of personal pronouns. 

    Even when the word order must be altered slightly, you can use the technique: 

    Mrs. Dimwit consulted an astrologer whom she met in Seattle. (She met him in Seattle.) 

    Jones is the man whom I went fishing with last spring. (I went fishing with him.) 

    Joyce is the girl who got the job. (She got the job.) 

    Whom can we turn to in a time of crisis? (Can we turn to her?)

    The delegates differed as to who they thought might win. (Not whom. Here the entire clause is the object of the preposition. Substitution is particularly helpful in cases such as this. They thought he might win.) 

    Who is that masked man? (subject)

    The men, four of whom are ill, were indicted for fraud. (object)  

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  And, now, for a really tough test (or, at least, most people trip up on it): 

    I decided to vote for whoever/whomever called me first. 

    Give it to whoever/whomever deserves it. 

      It's “whoever” in both cases. Even though you can read the first sentence as “I decided to vote for him” (which would make it “whomever”), the entire phrase “(he) called me first” is the object of the preposition “for.” So, it's “whoever.” It's the same for the second example: “...he deserves it” wins out.