“Relative Pronoun: A pronoun that alone or as part of a phrase introduces a relative clause: who in the man who came to dinner; on whom in the woman on whom I reply. The relative pronoun refers to an antecedent (the man/who, the woman/oh whom), and functions within the relative clause: as subject in who came to dinner; as complement of a preposition in on whom. There is a gender contrast between the personal set of who pronouns and the non-personal which pronoun, and there are case distinctions in the who set: subjective who(ever), objective whom(ever), genitive whose. However, except in a formal context, who(ever) replaces whom(ever). That can be used as a relative pronoun in place of who, whom, or which, except as complement of a preposition: the woman who/that I rely on, but only the woman on whom I rely. That can be omitted when functioning as object (a man that I know; a man I know), but not as a subject (a man that knows me). The omitted pronoun is sometimes referred to as a zero relative.”
Excerpted from: McArthur, Tom. The Oxford Concise Companion to the English Language. New York: Oxford University Press, 2005.
