whom 
/ˈhu:m/ pronoun
objective case of who /ˈhu:m/ pronoun — used in formal writing or speech ▪ To whom am I speaking? ▪ His brother, with whom he is very close, works for the same company. ▪ I was introduced to the artist, whom I was anxious to meet. ▪ an author whom I had never heard of
usage Whom is a more formal word than who and is not commonly used in ordinary speech and writing, where it can seem awkward and unnatural.
▪ (formal) To whom did you speak? = (formal) Whom did you speak to? = (more commonly) Who did you speak to? ▪ (formal) We weren't sure whom to hire. = (more commonly) We weren't sure who to hire. ▪ (formal) The person to whom we spoke to was very helpful. = (formal) The person whom we spoke to was very helpful. = (more commonly) The person who/that we spoke to was very helpful. ▪ (formal) Her brother, whom I met last year, is an attorney. = (more commonly) Her brother, who I met last year, is an attorney.
▪ (formal) To whom did you speak? = (formal) Whom did you speak to? = (more commonly) Who did you speak to? ▪ (formal) We weren't sure whom to hire. = (more commonly) We weren't sure who to hire. ▪ (formal) The person to whom we spoke to was very helpful. = (formal) The person whom we spoke to was very helpful. = (more commonly) The person who/that we spoke to was very helpful. ▪ (formal) Her brother, whom I met last year, is an attorney. = (more commonly) Her brother, who I met last year, is an attorney.
to whom it may concern — see 2concern
