Who, whoever, etc are nominative. Whom, whomever, etc are non-nominative. So its "Whoever called me STFU!" lol, but its "Call whomever you want" or "To whom it may concern" but not "To who it may concern."Originally Posted by ReDSanchous
But whom is becoming archaic, especially in casual conversation. In most cases you can use who in place of whom and it actually sounds more normal.