Start with the job the word does
Who and whom refer to people, but they do different grammatical jobs. Who is the subject form: it stands for the person doing the action or being described by the verb. Whom is the object form: it stands for the person receiving an action or following a preposition. Compare “Who called?” with “Whom did you call?” In the first question, the person called. In the second, you called the person.
A quick check is to answer the question with he or him. If he fits, choose who; if him fits, choose whom. “Who called?” becomes “He called.” “Whom did you call?” becomes “You called him.” You do not need to keep that answer in your writing. It is simply a way to hear the person’s role more clearly.
Look past the order of a question
Questions can hide the relationship between a verb and its subject or object. In “Who wrote the note?” the answer is “She wrote the note,” so who is the subject of wrote. In “Whom did the editor invite?” the answer is “The editor invited her,” so whom is the object of invite. The word did changes the order of the question, but it does not change who performed the invitation.
The same distinction appears in statements that add information about a person. “The reader who noticed the error wrote to us” uses who because the reader noticed the error. “The reader whom we thanked wrote to us” uses whom because we thanked the reader. Try restoring a simple clause—“the reader noticed” or “we thanked the reader”—before making your choice.
Check the right clause in longer sentences
A nearby verb can distract you when a sentence contains more than one clause. Consider “Who do you think will lead the discussion?” You think something, but the person in question will lead the discussion. Within that inner clause, the person is the subject: “She will lead.” Who is the appropriate form. By contrast, “Whom do you think the group will choose?” points to “The group will choose her.” There, the person is the object.
This is why a mechanical he-or-him swap at the start of a whole sentence can mislead you. Find the clause that actually contains the gap. For “the colleague who I believe is ready,” test “I believe she is ready”; the colleague is the subject of is. For “the colleague whom I believe you met,” test “you met her”; the colleague is the object of met. The words “I believe” do not decide the case.
Notice what prepositions do
A person following a preposition is its object. That makes whom the traditional choice in phrases such as “with whom,” “for whom,” and “to whom.” For example, “To whom should I address the letter?” puts the preposition to directly before whom. This arrangement is common in formal prose.
English also allows the preposition to move to the end: “Whom should I address the letter to?” In ordinary conversation, many speakers would say, “Who should I address the letter to?” Both forms communicate the question, though the first two preserve the formal object form. If you move the preposition to the front, use whom: “To whom should I address the letter?” Avoid treating whom as a general signal of politeness; in “Who is responsible?” the person is still the subject, however formal the setting.
Choose clarity as well as correctness
Whom remains useful when it makes a relationship easy to see, especially after a preposition or in carefully edited formal writing. Yet who often sounds more natural as an object in everyday speech and informal prose. “Who did you invite?” is unlikely to puzzle a reader. A consistent, readable sentence is usually better than one that strains to sound elevated.
When you are unsure, first identify the verb and ask whether the person acts or receives the action. Then test a short answer with he or him in the relevant clause. Finally, read the full sentence aloud for tone. If “whom did you invite?” sounds too stiff for the piece, “who did you invite?” may serve it better. If the sentence remains awkward, rewrite it: “Which guests did you invite?” Grammar gives you a useful distinction; editing lets you decide how best to express it.
