I am very uncertain about when to use "most of whom," "most of who," or "most of which."
Please give concrete examples instead of only rules like, "this is the subject, so you should..."
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"Who" is a subjective pronoun. It is used in the place of a subject in a sentence or phrase. For example "Who is coming to dinner?" ("Who" is the subject.)
"Whom" is an objective pronoun. It is used in the place of an object in a sentence or phrase. For example, "With whom are you coming to dinner?" ("Whom" is the object of the preposition, while "you" is the subject.)
Since "most of _____" is a prepositional phrase, the correct usage would be "most of whom." The phrase "most of who" should probably never be used.
Another way to think about the difference between the subjective/objective pronouns is to revise the sentence to include a personal pronoun and see which form (he/him or she/her or they/them) fit.
For example, take this sentence: "I have twelve co-workers, most of whom are British." Could you re-write it as two sentences: "I have twelve co-workers. Most of they are British?" Probably not! You would say "Most of them are British," because the "most of" prepositional construct takes an objective pronoun. I can't think of a case where it would take a subjective pronoun.
As to whether you would use "most of whom" or "most of which," both "who" and "which" are relative pronouns. "Who" is used to refer to people, while "which" is used to refer to animals and things. For example, "I have twelve co-workers, most of whom are French, and I have twelve wine bottles, most of which are empty."
Sometimes "which" might be used to refer to a group or crowd of people where individuality is less distinct. For example, "The crowd, most of which were local fans, cheered when the opposing pitcher got knocked unconscious by a line drive." I believe either "most of whom" or "most of which" could be used in that kind of sentence.
The who/whom distinction is covered elsewhere (in the "most of…" context it's whom, but in modern usage who is often used), but this question brings up the legitimate question of distinguishing between when to use who and when to use which. For this, simply consider whether the collection you're describing consists of people or not.
There were 10 postmen, most of whom wore hats.
There were 5 post boxes, most of which were empty.
This can be confusing, because in other contexts we use which for people as well as inanimate objects.
Which postman delivered your letter?