Relative pronouns   We use: -> who(m) that to refer to people -> which/that to refer to things -> whose with people, animals and objects to show possession.   ! Who, which and that can be omitted when they are the object of the relative clause: He's the man (who) I am working for.   !! Whom can be used instead of who when it is the object of the relative clause. Whom is always used instead of who or that after preposition. That's the girl to whom Billy got engaged.   !!! Who, ...