Regina, Saskatchewan; Canada
Yes, Pete said it well.
"Who was that masked man?
I received a call but I don't know from whom."
These are also good examples, I think, in how they are used best in interrogative manners and cases where there is uncertainy rather than a certain specification. Confusion caused by using them otherwise, is what causes me problems. But when mind them as interrogative (questioning) and uncertain, they seem to work best.
"I know not whom you serve."
"Whom do you serve? "
"He is who I serve" sounds terrible.
It is better to say, "him I serve/ I serve him." Or specifically "The king I serve/ I serve the king"
Another mistake I think is trying to use both at the same time:
"He whom I serve knows best."
"Him I serve knows best." is probably best.
A knowing is being shown, therefore it is not necessary to say "whom I serve knows best".
If it refers to an unknown person though, "whom" shall be much more relevent.
"Whom I serve in my next life time"
Hope that makes sense