General Rules:

  • If you can replace the pronoun with “he,” the correct choice is “who.” If using “he” is awkward, “whom” is the correct choice. Do the same trick using “they” for plural constructions.
  • When the preposition “to” precedes the pronoun, “whom” is the correct choice.

Who


Rule: Use this pronoun when referring to humans and animals with a name. It’s always the subject of a sentence, clause or phrase and never the object.
  • The boy who fell during recess appears to be unharmed.
  • Who is knocking on the door?


Rule: Offsetting commas are used for nonessential clauses starting with "who" and omitted for essential clauses.

  • Her mother, who is also a professor of law, has law licenses in three states.


Whom


Rule: Use this pronoun when someone is the object of a preposition or verb.

  • The doctor to whom she was referred specialized in rare diseases.
  • Whom do you wish to ask?