1. Who = relative pronoun for the subject of the sentence 2. Whom = relative pronoun for the object of the sentence 3. Whose = relative pronoun indicating possession ( 'di chi' in italiano) For example: 1. That's the girl who won the writing prize. 2. That's the girl whom I saw at the prize-giving ceremony. 3. That's the girl whose book won the prize. If you remember that 'whose' means 'di chi', and is always followed by the thing which is possessed (whose book, whose house, whose friend etc), you shouldn't have any problems with this word. It's also worth noting that while sentence 2 is technically correct, native speakers rarely use 'whom' except in formal situations. We would be more likely to say 'That's the girl who I saw..' or 'That's the girl that I saw' or - most commonly - 'That's the girl I saw'. The only time when it is necessary to use the object case relative pronoun 'whom' is after a preposition (with, to, of , from) in the following type of sentence: My grandfather is the person of whom I have the fondest memories. This construction is typical of written and more formal language.

Have a look at this page for an explanation: http://www.englishpage.com/minitutorials/who_whom.html