Who and Whom
Although the making of case was lost in the English nominal system nearly 1,000 years ago, one still distinguishes between subjective and objective case in the pronominal system: “I” vs “me”, “he” vs “him”, “she” vs “her” … Only the second-person “you” has just one form. So it might seem logical that this distinction should also be observed for the personal interrogative/relative pronoun “who/whom” as well. But this is not the case. “Who” has clearly won the day, even when it functions as an object. For most native speakers, the form “whom” hardly exists. In general, it is used in formal English, and even there, it would be absurd to insist on “whom” in all cases.
Sentence (a), for example, is, at best, extremely formal-sounding. It’s hard to imagine that someone would actually say “whom”, rather than “who”, here in a conversation:
a) Whom did you see on your way to work this morning?
There is really only one case in which the use of “whom” is natural : when it is the object of a preposition that immediately precedes it, as in (b):
b) With whom were you planning to collaborate?
In fact, it’s an understatement to say that “whom” is natural here: it is the only option it would be considered grammatically incorrect to use “who” in stead of “whom” in these examples. Of course, sentences such as these are somewhat formal for an entirely different reason: the prepositions are “fronted”, that is, moved to the front of the clause. In conversation, it is far more common to “stand” the preposition :
c) She is the only person alive who I am not willing to talk about.
Who were you planning to collaborate with?
One also encounters the structures of (c) with “whom” instead of “who”, but this is stylistically a bit odd, since it mixes a formal element (“whom”) and a less formal one (preposition stranding). There are some cases where the preposition cannot be stranded, most notably with partitives such as “some/all/many/two of whom”:
d) There were 20 people present, some of whom I had never met.
There is another syntactic environment in which you will sometimes find people using “whom” instead of “who”. Consider the following example :
e) Miller, who/whom she thought was her friend, actually tried to kell her.
At first glance, it looks as if the relative pronoun is the object of “thought”. But oon closer inspection, we see that it is the subject of a tensed clause embedded under “thought” (logically : “he was her friend”). In other words, the object of “thought” is not “who/whom” but the entire clause. Traditionally, “whom” was considered a grammatical mistake here, since it is logically a subject. But one does encounter sentences such as (e), use “who” and not “whom”.