Language/German/Grammar/Be-Polite
Well, I guess every foreigner has heard about that famous German formal version of 'you'.
The informal translation is 'du', which is used when you're talking to family members, friends, basically people you're close to or when you talk to other adolescents or children.
When you're talking to an authority person, like adults, officers and teachers, you must say 'sie'.
So when you're talking to your classmate you say 'Kannst du mir helfen?' (Can you help me?) and when you're talking to your teacher you say 'Können sie mir helfen?'. Both has the same translation.
When you can't say for sure if the person you're talking to is the same age as you are or maybe already over 18, you should better say 'sie' because if the person really is older than 18 he or she would feel offended.
Another point is writing formal letters and e-mails.
The formal 'you' is written 'Sie' and the informal 'you' can be written 'du' but also 'Du'.
The same goes for the formal way of 'Ihre' (your), 'Ihr' (your) 'Ihnen' (you) and 'Ihren' (your).