Two forms of one word
Who and whom are the same pronoun in two cases. Cambridge describes whom as the object form of who, used when the person is the object of the verb. It works like the difference between he and him, which the Style Manual calls the subjective and objective case.
Both words do two jobs. The Style Manual lists them among the relative pronouns, which refer back to a noun already known from the context, and among the interrogative pronouns, which ask questions.
The person who wrote the report has left. (relative)
Who left the lights on in the office? (interrogative)
Setting the check, step by step
When you do want the strict form, this check finds it. It is a way of applying the case rule, not a rule of its own.
Find the clause the word belongs to. In “The editor ___ we hired starts Monday”, the clause is “___ we hired”.
Answer it with he or him. Turn the clause into a plain sentence: “we hired him”, not “we hired he”.
Match the ending. Him and whom both end in m. If the answer is him, the strict form is whom; if it is he, it is who.
So: “The editor whom we hired starts Monday”, but “The editor who hired us starts Monday”.
After a preposition, whom. When the word comes straight after to, for, with and so on, it is the object of the preposition. Cambridge notes that whom is commonly used with prepositions.
The trap is a clause inside a clause. In “the editor who I think hired us”, the words “I think” sit in the middle, but the pronoun is still the subject of hired, so it stays who. Lift out the interruption before running the check.
What current usage guides say
Here the dictionaries are plain. Cambridge says whom is used in formal styles or in writing, that we don’t use it very often, and that it is more common in writing than in speaking. The Oxford Learner’s Dictionary marks whom as formal, and its grammar note says it is not used very often in spoken English, where who is usually used as the object pronoun, especially in questions.
In other words, who in an object position is ordinary English in speech. Whom is a choice of register: right for a formal letter or a report, stiff in a chat.
Where the formal and everyday forms part ways
| Situation | Formal | Everyday |
|---|---|---|
| A question about the object | Whom did they invite? | Who did you invite to the party? |
| After a preposition | To whom should I address the letter? | Who should I address the letter to? |
| Reported question | He asked me with whom I had discussed it. | He asked me who I had discussed it with. |
| Defining relative clause | The family whom I met at the airport were very kind. | The family (who / that) I met at the airport were very kind. |
All four pairs are from the Oxford Learner’s Dictionary entry and grammar note for whom, which calls the formal pattern after a preposition “very formal”.
Defining and non-defining clauses
The Oxford note draws one more line. In a defining relative clause, the kind that tells you which person is meant, whom is not often used: you can use who or that, or leave the pronoun out altogether. In a non-defining clause, the kind set off by commas, you use who or, more formally, whom; you can’t use that, and you can’t leave the pronoun out.
Our doctor, who/whom we all liked very much, retired last week.
That matches the Style Manual’s advice on that and which: commas mark information that is not essential, and the punctuation is what makes the meaning clear.
Whom in fixed phrases
A few patterns keep whom even in fairly plain writing, because it follows a preposition inside a phrase. Cambridge gives “many of whom” as an example: “Over 200 people attended the ceremony, many of whom had known Harry as their teacher.”
Choosing for your reader
Since both dictionaries tie whom to formal writing, the question to ask is less “which is correct?” than “who is reading, and how formal is this?” There is also a third way: turn the sentence around so that neither word is needed.
To whom should I send the form? (formal)
Who should I send the form to? (everyday)
Where should I send the form? (no pronoun needed)