“Who” and “whom”’ cause all sorts of problems for writers. No one seems to know when to use which one, and whether to even bother with “whom” at all. More on that in a minute.
“Who” is a subject pronoun. It is the subject of a verb, even if that verb is in a dependent clause.
“Whom” is an object pronoun. It is the object of a verb or a preposition.
Substitute “he” or “him” to determine whether to use “who” or “whom.” If “he” makes sense, use “who.” If “him” makes sense, you can use “whom” (both have an M).
– The employee, who/whom the boss promoted after only six months, ended up doing well in her new post. (The boss promoted HE? No, the boss promoted HIM = whom)
– The employee, who/whom everyone said was incompetent, got promoted after only six months.
(Everyone said HIM was incompetent? No, everyone said HE was incompetent = who). This one is wrong a lot — editors change a lot of overcorrected “whoms.”
Usage of “who” and “whom” is in transition, and “whom” is dropping out of English.