А как знать, что можно и "на ты" с каким-то человеком? Если например мы с ним до сих пор на вы, когда заменить обращение к нему?

lobitel_rus, it was a nice thought to ask about that, as neither I know when to use each of them properly. You are right, in spanish there is also such a distinction, yet, in english things are much easier... you see both second person plural and singular coincide
I have always wondered how one can tell whether someone addresses to him (in second person) in a polite manner or not

Anyway, I have noticed, that russians in general use by default "Вы" towards strangers.
I guess I have to practice second plural more...