Who/Whom

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Quick Tip:
Think about "he or him" and listen for the 'm' sound.

Longer Explanation:

• If it’s a question, answer it with he or him.
  Did you see who/whom picked up my notebook?
  He picked it up. ➜ ​Did you see who picked up my notebook?
• If it’s not a question, ignore the first half and substitute he or him in the sentence.
  I'm not sure who/whom I'm going to vote for.
  I'm going to vote for him. ➜ I'm not sure whom I'm going to vote for.