originally: whom was the form of who that you would use if who was the object in a sentence - the direct object or the indirect object whom did you hit? of whom did you think? But for about a hundred years that form has been dying. So now we really say: who did you hit? who did you think of? So, unless you want to get crafty with your English, you can forget "whom" Whose however still does exist. It stands for "of whom" but only when the "who" owns something. So.... Whose book is this? (Of whom is this book - but note NO ONE says "Of whom is this book", but that is what it stands for. So it's that easy: FORGET "WHOM" "WHOSE" means ---- well it is better to think of that one example "Whose book is this?" which means "Who owns this book?" Does that make sense?

whom = a quien whose = de quien, cuyo(s)(a)(as); lo/la/el que

"Whose" indicates possession. Ex. A: Whose book is that?" B: It is mine. Technically "whom" is used instead of "who" to represent the object of a sentence (ie. in the accusative case), but in modern English "who" is almost always used instead of "whom", even in formal writing. English isn't as big on cases as other European languages and I imagine Spanish probably has equivalents of "who" and "whom" in different cases. In modern English, we generally just use "who" for all cases.

Whose: the possessive form of who and which - "the boy whose mother works in a factory"; "the tree whose branches fell on the car". Whom: the objective form of who - "The man whom we met last week"; "the person with whom we spoke". Note that many English speakers avoid using "whom" when speaking although they may use it in formal writing. You could just as well say - "the man who we met" or "the person who we spoke with" (but not "the person with who we spoke" )