It's a bummer that when I learn a rule, I can't let it go. One example is the usage of
whom. For a long time, I had no idea when I was s'posed to use it, and when
who would be more appropriate. Then I learned about noun declension in German and Latin, and badabing!; since then I've corrected every
who/whom mistake I have seen; sometimes in my head, sometimes out of it, always with a pang of embarrassment that I can't let go.
By the way, if you ever have a question as to which you should use, just ask yourself: "Is
who/whom referring to the subject of the sentence?" If so, use
who; otherwise, use
whom.
I don't know where
whom /hūm/ came from. I know that in German, the
Dative form of
wer (which means "who") is
wem (which means "to whom"). Wikipedia will tell you
more, I'm sure.