A trick I was taught to tell which to use: reword the sentence using they/them, and determine which sounds correct. If “they” sounds better, then “who” is correct in the original sentence. If “them” sounds better, “whom” is correct.
Example: “Who ate my Teamster sub?” Which of these two sounds better:
- “They ate my Teamster sub.”
- “Them ate my Teamster sub.”
In this case, we should use “who”.
Example: “Who did you see Hot Tub Time Machine with?”
- “You saw Hot Tub Time Machine with they.”
- “You saw Hot Tub Time Machine with them.”
In this case, it is “whom”.
Though even more proper would be “With whom did you see Hot Tub Time Machine?” because
This is the sort of nonsense up with which I will not put!
The way I learned it is that “whom” is to “who” as “me” is to “I”.
Granted, there’s some cases where “me” is incorrectly used but sounds more correct. e.g. “You’re smarter than me” which sounds correct, but should actually be “You’re smarter than I” due to the implied verb (am) which is omitted at.
