Last month I taught a writing workshop for a public relations firm in Cambridge, Mass. The principal there invests in professional development for her employees, and fine-tuning writing technique is part of that effort. It was a great session of discussing challenges, reviewing press releases and sharing ideas.
One of the copy writers there asked about the difference between “who” and “whom.” Though I know what “sounds right,” I couldn’t think of how to explain it; it had been so long since I learned the rules myself.
I turned to Grammar-monster.com:
“The word who can only be used when it is the subject of a verb. That might sound confusing, but it just means it is like the words I, he, she, we, and they. Just like who, each of these words can only be the subject of a verb. The difference with who is that some people are unsure when to use who and whom.”
Examples:
Who paid for the meal? 
(Who is the subject of the verb to pay)
Sarah gave the tickets to who? 
(Sarah is the subject of to give, but who is not the subject of any verb. Therefore, you cannot use who. It should be whom.
The best part of collaboration is learning from each other. Reviewing the “who” and “whom” rule — that “who” must be a subject of a verb — proved beneficial to us all.