Who vs. Whom

Mary Norris explains the classic grammar mixup of who vs. whom:

My test for the correct use of “who” or “whom” in a relative clause—“who I know will use it judiciously”—is to recast the clause as a complete sentence, assigning a temporary personal pronoun to the relative pronoun “who/whom.” “I know she will use it”? Or “I know her will use it”? No native speaker of English who has outgrown baby talk would say “her will use it.” The correct choice is clearly “she”: “I know she will use it judiciously.” If the pronoun that fits is in the nominative case, acting as the subject (“I,” “you,” “he,” “she,” “it,” “we,” “you,” “they”), then the relative pronoun should also be in the nominative case: “who I know will use it judiciously.” Yay! I got it right.

Suppose I had written that I turned over the comma shaker to a colleague who I have known for years. Recast the relative clause as a complete sentence with a personal pronoun: “I have known she for years”? Or “I have known her for years”? This time the correct choice is “her,” which is in the objective case (“me,” “you,” “him,” “her,” “us,” “you,” “them”); therefore the relative pronoun should be in the objective case (“whom”). I should have written, “I turned over the comma shaker to a colleague whom I have known for years.” Boo! I got it wrong.