Most of the time, you're supposed to use who in writing...but there are those rare sentences when you're better served with a fancier-sounding whom instead. It's difficult for many writers to know when to use which, but get it wrong and suddenly every reader is a language expert. Know when it's necessary to use whom, and you'll always sound like you know just what you're writing about.
The Subject-Verb Love Affair
To know when a whom is called for, first you've got to know how to identify the subject of your sentence. It's difficult, because who and whom are both pronouns -- but confusingly, they aren't both subjects.
You'll know it's a subject if the verb depends upon it to function. For example, if I write Sally searched for the answer online, the word searched is the verb. But without Sally, nothing makes any sense. Just try it: Searched for the answer online. Who did? Sally did; and that's the subject of the sentence. Police solve crimes by eliminating suspects. Writers solve word problems by eliminating subjects.
Who or Whom?
Look for the suspected subject in a sentence, and eliminate it to determine whether you should be using who or whom.
Who is looking for you?
In the above sentence, is who the subject, or is it you? Let's find out. Eliminate you first, and re-read the sentence: Who is looking? Still makes some sense, right? Try it with who next: Looking for you? But this time, it doesn't really make sense. This means that who is the subject. Whenever who is the subject, it's always right.
But you're supposed to use whom when you're talking about an object.
Tell whomever you like.
Tell is clearly the verb in the above sentence, because it's the action word -- it's the word that's making something happen. But whomever isn't telling anyone anything -- you is the one doing the telling. Whomever is who it's being told to -- and that makes it the object, not the subject, of the sentence.
Whom did you say is calling?
How do I know that I'm supposed to use whom and not who, in the above query? Simple -- because of you. Without it, the sentence has no subject: Whom is calling can't be used. The word whom is never a subject. You can always change the sentence to Who is calling, but not if you comes along for the sentence. Who did you say is calling is incorrect, because the sentence can only have one subject -- and you is that subject.
Figured it out yet? Make sure your whom-who decision-making is perfect with this fun Who vs. Whom quiz.