Quote Originally Posted by Antonio1986 View Post
Back to Genetive.
When someone enter in the shop I say to him:
Вы хотите / Вы хотели бы воду/пиво/вино?
They told my that is better to say: Вы хотите / Вы хотели бы воды/вина/пива?
The problem is that I don't understand whether this is Roditel'nyi or just Iminitel'nyi / Mnozhestvenoe chislo.
Yes, it is Genitive (родительный), but here you see its another usage - the so-called "partitive". Actually, there are traces of partitive in modern Russian where its form is slightly different from genitive, e.g. чай (nominative) - чая (pure genitive, as in "стакан чая", or "у меня нет чая") - чаю (partitive: Я хочу чаю - I would like some tea). The meaning of partitive is to convey the idea of "some": not all the tea, but just a limited amount of it. More details on this: Partitive - Wikipedia, the free encyclopedia.