Anytime I can work an OutKast reference into a ChanteSez, it’s a good day. And yes, I know that the song title is actually “Hootie Hoo.” But that’s beside the point … kind of like asking which is the better group: OutKast or A Tribe Called Quest.
(Leave me a comment if you can call it. I couldn’t when my cousin put the question on Facebook.)
So, on to today’s lesson. What’s the difference between “who” and “whom”? To state the obvious, the letter “m.” And that’s the key to remembering when to use these pronouns.
Replace the who/whom with the word “him.” Notice that both “whom” and “him” end with the letter “m.” If “him” fits, use “whom.”
So remember: Him = whom.
Here’s the tip in action. You’ll have to rephrase the sentence in your mind for this tip to work, but it’s easy.
For whom the bell tolls.
- Rephrase: Who does the bell toll for? It tolls for “him,” so “whom” is correct.
To who should I give the ball?
- Rephrase: Should I give the ball to “him”? Yes, so “whom” is the word to use here, not “who” as shown incorrectly above.
He said the woman with whom he traveled was a relative.
- Rephrase: He said he traveled with her, a relative. Since “her” is the feminine pronoun sibling of “him,” “whom” is correct here.
ChanteSez.com 
One of my favorite rules! But I would tweak what you said just a little bit. Try to replace who/whom with “he” or “him”. TRY BOTH. One of them will clearly sound wrong.
You say to try “him”, which is right, but to be more complete one should try “he” and “him” and that will provide a slamdunk case for “who” or “whom”. Some people might not understand the rule without the “he” case, because that will show them when “who” is actually correct.
What’s next, “me” vs “I”? 🙂
Good tip, Chris! I think you’ve clarified things quite well in your comment. Thank you for leaving it!