Who is always the subject of a sentence, and whom is always the object of the sentence, a verb, or a preposition.

Here’s how you know which to use:

1. Always start with the words following (containing) who or whom to decide which to use.

2. First, try a simple substitution: he for who and him for whom.

3. If that substitution doesn’t work, rewrite the clause to help you see which pronoun to use. But still start with the words following who or whom.

The examples below come from my Brush Up on Your American Grammar Skills program and are what we usually go over in class. You might want to take a stab at each before reading answers and explanations.

1. May I tell her (who/whom) is calling?
2. You may vote for (whoever/whomever) appeals to you.
3. You may vote for (whoever/whomever) you wish.
4. I will speak to (whoever/whomever) answers the phone.
5. I will speak to (whoever/whomever) you suggest.
6. (Who/Whom) were you talking to?

Answers:

1. Who. Check out the words following who/whom: what do you need right in front of “is calling”? You need a subject for the verb “is calling.” You would say “HE is calling.” Therefore, the right answer is who.

2. Whoever. The same rule applies here: HE appeals to you. 

3. Whomever. Remember the rule: START with the words that follow who or whom. In #2 and #3, the first few words are the same but they change AFTER who or whom. And making that simple substitution doesn’t work. In this case, first use the words following who or whom and then continue with first few words of the sentence to find the answer.

You may vote for (whoever/whomever) you wish. You wish you may vote for (him/he). You’d choose him, right? That shows you that whomever is the right choice in this sentence.

Still with me?

4. Who. The same thinking shown in #1 and #2.

5. Whom. Again, making a simple substitution will not work, so start with the words following who/whom — you suggest — and ask “you suggest . . . what?” You suggest I will speak to him.

BUT / BUTT

6. Whom. Here’s where grammar and common usage butt heads: in terms of grammar, the answer is whom (Were you talking to him?), but honestly, it’s pretty hard to actually ask, “Whom were you talking to?” This is one place where the wrong word would work better than the grammatically correct one, at least in regular conversation or casual writing.

Now before you pull your hair out and do the “I don’t wanna learn this stuff” dance, keep reading. I advise my students to use “who” in regular speech. Why? Because no one knows the difference anyway, and it’ll pass over almost everyone’s head. It’s unremarkable, so it will allow your conversations to keep going. And if you use “whom” in speech — even correctly — you’ll stop most conversations cold. It doesn’t sound right even when it is!

But. You knew there was a but, right? In writing, it’s presumed that we all have time to think about what words to use, so I suggest that you take the time when you write — especially when it’s important that you get things right! — to figure out which word you need.