When do I use "whom"?
Use whom when the pronoun is the object of a verb or preposition — when something is done to it.
Contexti
If you picture the sentence as an arrow, who sits at the tail (the doer) and whom sits at the head (the target). Cover letters, formal emails, and legal prose are the places where getting this right still matters; novels and news writing have mostly relaxed.
A little moreii
After prepositions (to, for, with, of), use whom: "to whom it may concern." As the object of a verb, also whom: "whom did you see?" In ordinary speech, most people reach for who in both slots — but careful prose still preserves the distinction.
Examplesiii
For who should I leave this message?
For whom should I leave this message?
After the preposition *for* → *whom*.
Who did you invite to the wedding?
Whom did you invite to the wedding?
You invited *him* — object slot → *whom*.
Watch foriv
In spoken English, almost every whom slot is filled by who without anybody noticing. The distinction is a written-prose rule more than a speaking rule.
The full entryv
Subject versus object — the pronoun doing it versus the pronoun it happens to.