Who is a subject pronoun — use it wherever you would use he, she, or they. Whom is an object pronoun — use it wherever you would use him, her, or them. The fastest way to choose: answer the question with he or him. If him fits, use whom (both end in m); if he fits, use who.
So it is Who called? (he called) but Whom did you call? (you called him). That single test settles almost every case you will ever meet.
English pronouns come in two forms. Subject pronouns do the action of the verb; object pronouns receive it or follow a preposition. Who and whom are simply the question-and-relative-clause versions of that same split:
You already make this switch without thinking. You say he left, not him left, and I saw him, not I saw he. Who and whom follow exactly the same logic: use who when the word is doing the verb, and use whom when the word is on the receiving end of a verb or sits after a preposition such as to, for, with, or from.
When you are stuck, turn the question into a plain statement and fill the gap with he or him:
The memory hook is built in: whom, him, and them all end in m. If the answer ends in m, so does your question word.
Who — the subject, doing the verb:
Whom — the object of a verb or preposition:
Interrupting phrases like "do you think." In Who do you think will win?, the answer is he will win, not you think him — so who is correct. The phrase do you think is only an aside; ignore it and the subject who is obvious. Writing whom here is an error.
The "shall I say" trap. The polite telephone line is Who shall I say is calling?, not whom. Who is the subject of is calling (he is calling); shall I say is inserted in the middle. Reaching for whom to sound formal is a classic case of hypercorrection — overcorrecting yourself into a mistake.
Whoever vs whomever. The same rule applies. In Give the prize to whoever answers first, use whoever: it is the subject of answers, and the whole clause — not the single word — is the object of to. But in Invite whomever you like, use whomever: you like him, so it is the object.
Whom is at its strongest right after a preposition: to whom it may concern, the colleague with whom I travelled, a leader in whom we trust. In these fixed, formal patterns, who genuinely sounds wrong to most readers.
You can also move the preposition to the end, which is perfectly grammatical despite the old myth that a sentence must never end with one. Whom did you give it to? is correct, and in everyday English Who did you give it to? is what most people actually say. Stranding the preposition is fine; it simply tends to pull who along with it.
Honestly, the who/whom divide is about formality, not nationality. American and British English follow the identical grammatical rule, and both are quietly retiring whom in speech and casual writing. Who are you going with? raises no eyebrows on either side of the Atlantic.
Where whom survives — again in both varieties — is formal writing, legal and business correspondence, and set phrases after prepositions. So the practical guidance is simple: in conversation and informal messages, who is safe almost everywhere; in formal or edited writing, use whom for objects and after prepositions and you will read as precise rather than stuffy. What to avoid in every register is hypercorrection — dropping in whom where who is the real subject.
Rephrase the sentence and answer it with he or him. He gives you who; him gives you whom. Remember that whom, him, and them all share a final m, and you will get it right far more often than most writers do.
"Whom do you trust" is the strictly correct form, because whom is the object of trust — you trust him, and him and whom both end in m. That said, "who do you trust" is extremely common and sounds perfectly natural in everyday speech and writing. Save the whom version for formal contexts where precision is noticed.
Always "to whom." Prepositions such as to, for, with, and from are followed by the object form, and whom is that form — you give something to him, not to he. So write "to whom it may concern" or "the person to whom I reported." Casually, people say "who did you give it to," which is accepted informally.
Not obsolete, but clearly in retreat. In casual spoken English, who now covers nearly every position and few people notice. Whom survives mainly in formal writing and directly after prepositions, as in "for whom" or "to whom." Using it correctly still signals careful writing, so it is worth knowing — just don't force it where who belongs.
Answer the question with he or him. If him fits, use whom — both end in the letter m. If he fits, use who. For example, "____ did you call?" becomes "you called him," so it is "Whom did you call?" The same works with they and them: if them fits the answer, whom is your word.
"Who shall I say is calling" is correct. Who is the subject of is calling — he is calling — while "shall I say" is just an inserted aside. Switching to whom here is a well-known hypercorrection: people add it to sound formal, but it is grammatically wrong. Ignore the interruption and the subject who becomes obvious.
