As a follow-up, anyone familiar with the German language should find this simple. The use of 'whom' is in the
Accusative (Akkusativ auf Deutsch) case. When one would use 'mich' instead of 'Ich', 'dich' instead of 'du', 'ihn' instead of 'er', and so on (und so weiter), you use 'whom' (or 'wen', in German).
Same with the
dative (Dativ auf Deutsch) case (To whom, or 'wem'). Since this is quickly turning into a treatise on German grammar, I shall quickly stop, as German speakers no doubt should be familiar with to him (ihm), to her, to it, etc.
So, in summary - if it's
accusative or
dative, it's whom, or to whom, respectively.