Repeat after me: “English is a living language.” As such, it is constantly metamorphosing. And though people like you and me do our best to stem an inexorable tide of change long enough to preserve high standards for the sake of clarity, we have to know when to get out of the way and let the language evolve.
That transformation is occurring as, with the mother tongue’s permission, one word steadily wrests responsibilities away from another. The bureaucracy of English has given who the equivalent of a sleek corner office, while whom is relegated to what in the language world passes for a shabby battleship-gray desk in the boiler room.
Here is the status quo about the who/whom issue in formal writing:
Who and whom are in a class of words (also including which and that) called relative pronouns, because they relate the subject to the direct object: “Who did that?” “He is the one who did that.”
Who is relative to the subject of a sentence; whom relates to the object: In “She met him,” she is the subject, the person doing the action, and him is the object, the person having the object done to them.
At the same time, who and whom are interrogative pronouns, meaning that they are used in interrogations, or questions. (The other four words in journalism’s alliterative starting five — what, where, when, and why, plus a few others, are also part of this club.) So, if you convert a declarative sentence (a statement) to an interrogative sentence (a question) about the subject, you replace the subject’s pronoun, she, with another pronoun appropriate for the subject, who: “Who met him?” The answer, “She did.”
However, if you convert a declarative about the object to an interrogative about the object, you replace the object’s pronoun, him, with another pronoun appropriate for the object, whom: “She met whom?” The answer: “Him.”
(Some people use the letter that ends both whom and him as they key of a mnemonic device: He = who; him = whom.)
But don’t be fooled into thinking that whom is used only at the end of a sentence: The meaning of “She met whom?” is also conveyed with the wording “Whom did she meet?” There’s also the form “To whom was she introduced?”
Complicated? Definitely. And just as water seeks the lowest level, language searches for the easy way. That’s why, increasingly, whom is replaced in most usage by the “incorrect” who — the only situation in which it doesn’t work is the “To whom” form referenced just above, which is easily circumvented by “Who was she introduced to?” (Remember, sentences are permitted to end a preposition with.) The upshot is that the old guy in the boiler room doesn’t get many visitors, while the corner office is Party Central.
But take care, because formal writing is drawing out the slow death of whom; many writers and editors, I believe, are concerned about seeming sloppy if they retire it, so I expect it will continue to poke its head out the boiler room door and scowl at us for some time to come.