"You are a man."
Translation:Du er en mann.
The formal you ("De") could in theory be used, but it is very seldom used in Norwegian (unlike French and German), so with this kind of sentence it is hard to think of a situation where it would be natural.
I guess they mean "Thou are a man." Something like Sie in German, usted in Spanish?
There's nothing like Sie or usted, but there's something like thou or Ihr: De. And by that I mean that it's an archaic word never used except if you wanted to act like you were living a hundred years ago. In that case you could say "De er en mann", but as this is so uncommon it is not accepted as an alternative.