Which floor does he live on?在这句中which 是代词,在句中做介词on的宾语,动词live是不及物动词没有宾语所以要加介词才能跟宾语.Where does he live在这句中where 是副词在句中做状语,因为动词live是不及物动词后不跟宾语能跟状语.