Check out A Tricky Grammar Construction by rogersgeorge. Here is an excerpt:
Regular readers (all three of you) know that I mention “whom” a lot. —Because a lot of people get it wrong, harrumpf.
Today I offer a vote against “whom”!
Review: Use “whom” when you have an object. Think of the stuffy
To whom it may concern
That’s correct. “Whom” is the object of the…