How to use who and whom correctly: sentence examples (with an explanation)
If you can replace a word with “he” or “she” then use "who". However, if you can replace it with “him” or “her,” use "whom". See the following sentences.
This is who warned me. (It is He/she warned me. Not “him/her” warned me)
Jack is the one who wants to go. (He/she wants to go. Not “him/her” wants to go)
With whom am I speaking? (I am speaking with him/her. Not I am speaking with he/she)
To whom this may concern. (This concerns him/her. Not this concerns he/she)