Rules for writers, Tenth edition - Diana Hacker, Nancy Sommers 2021

Use who and whom correctly in subordinate clauses
Distinguish between who and whom
Grammar

The choice between who and whom (or whoever and whomever) occurs primarily in subordinate clauses and in questions. Who and whoever, subjective-case pronouns, are used for subjects and subject complements. Whom and whomever, objective-case pronouns, are used for objects. (see 25a and 25b.)

An exception to this general rule occurs when the pronoun functions as the subject of an infinitive. (see 25c; see also 24e.)

25a Use who and whom correctly in subordinate clauses.

When who and whom (or whoever and whomever) introduce subordinate clauses, their case is determined by their function within the clause they introduce.

In the following two examples, the pronouns who and whoever function as the subjects of the clauses they introduce.

When functioning as an object in a subordinate clause, whom (or whomever) also appears out of order, before the subject and verb. To choose the correct pronoun, you can mentally restructure the clause.

When functioning as the object of a preposition in a subordinate clause, whom is often separated from its preposition.