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