Diamond: you are sooooo close! It would probably be best to say: - He fell in love with a girl that he didn't know. This is closest to what you have written and is probably acceptable. We would use the past tense since now that he has fallen in love with her he obviously knows her. - He fell in love with a girl whom he didn't know. This is probably the best way of saying it - I hope :))

Both are fine.

diamond is right regarding didn't rather than doesn't. i would just add - the word THAT is implied in the second sentence.

Ralph's completely right in my view of its context

the two sentences are fast as you