Having trouble with who vs. whom?
The Immigration Daily has a lesson, better written than most, on telling the two apart, including many examples and a quiz.
It also shares the rule most of us use:
If you can substitute him, then use whom; if you can substitute he, then use who.
Consider this statement: The salesperson whom you requested is away. (You requested him, so use whom.) Note that whom is the object of the verb, not the subject, in the subordinate clause whom you requested. Let's change the sentence slightly: This is the salesperson who won the award for most cars sold this month. Who is now the subject of the clause who won the award. Substitute he for who, and the sentence makes sense.