I believe I’ve mentioned before that, in speaking at least and often in writing, using who exclusively and forgetting whom exists will work just fine.
However, the other day I listened to a radio ad about an online dating service in which a woman extols the qualities of her new boyfriend “who I met online,” or words to that effect.
This grated on my ears. Why?
The answer is that this sentence calls for whom, not who. She met her boyfriend online, and thus boyfriend is the object of the sentence, necessitating the objective use of whom.
Since it’s a commercial, probably no one cares (or knows the difference) except for a few grammar jerks like myself. Nonetheless, it’s worth noting.
Here, then, is an easy tip for determining when to use who and whom. Invert the sentence and see if he or him works better. For instance, you have a test question to choose between who and whom like this:
Who/whom should I thank for this?
Invert the sentence: I should thank him for this.
Just as you can’t use he in this active sentence, you can’t use who in the question. Tuck that grammar tip away for future use.
(Sorry, gender usage watchers, this trick just works better with he and him than with she and her since it gives you the m when you need it.)