So I was going to give you a section of my favorite grammar book, but it was very complex, so I turned to Grammarly, which dumbed it down for me in the same way my grammar professor did (yes, there are professors who teach grammar among other things -- at a fast pace -- and are annoyingly exceptional at it).
So here's what Grammarly said:
"Who should be to used to refer to the subject of a sentence.
Whom should be used to refer to the object of a sentence."
Try this simple trick: If you can replace the word with he or she, who is your best bet. If you can replace it with him or her, whom is the right choice. (Memory device: him ends with m, just like whom.)
Who went to the park? (He went to the park. She went to the park.)
Whom should I go to the ball with? (I should go to the ball with him or her.)
Grammar Girl gives a more in-depth version if you are interested.
So here's what Grammarly said:
"Who should be to used to refer to the subject of a sentence.
Whom should be used to refer to the object of a sentence."
Try this simple trick: If you can replace the word with he or she, who is your best bet. If you can replace it with him or her, whom is the right choice. (Memory device: him ends with m, just like whom.)
Who went to the park? (He went to the park. She went to the park.)
Whom should I go to the ball with? (I should go to the ball with him or her.)
Grammar Girl gives a more in-depth version if you are interested.