The pronoun who, in the English Grammar, is the interrogative and relative pronoun, used to refer to humans. wikipedia

Other pronouns in these groups ( which, that, what, whatever, whichever, etc) do not change forms across case, person, gender, and number, but who does change form with case.

  • who - Nominative / Subjective
    • Who took my car?
    • I want to know who did that.
  • whom - Objective
    • With whom are we meeting?
    • To whom do you wish to speak?
  • whose - Possessive
    • Whose car is it?
    • I forgot whose phone it was.