If it helps ...
Use 'whom' when you would say 'him'.

So in your example (not particularly good example to use, but you might have to re-phrase the statement), "To whom are you writing a letter?" "I am writing a letter to him".

So in this case it is whom.

A counter-example would be "Who threw the ball?" "He threw the ball" in this case it is who.

"To whom did you throw the ball?" "I threw the ball to him" ...

So if you would use HIM, use WHOM.

I hope that helps clear up your quandry without the technical jargon.
( Ghoti Fingers, Wed 16 Jul 2008, 11:32, Reply)