when do you use whom?

[warthog]

what's the rule?

to whom it belongs?

four people, all of whom...?

argghgg i must have been asleep when they covered this.

It's used when it's the object of the sentence, as in your example. I think it may die out in much usage, though.

This has useful examples.

thank you! got it now.

That would be great, amicissima! And then continue the conversation in manner of Jane Austen...