I need to analyse some structures

Is this homework then? We won't do homework for you.

But I will briefly answer your second question:

Can I say that Roger is the subject? If not, what is Roger in this clause?

Roger is not the subject structurally, although it refers to the same person as the subject. In that sentence, Roger is a vocative (addressing term), not really part of the core of the sentence (probably a sentence-level adjunct-- that is, like an adverb). The subject of imperatives is a separate topic, and possibly one you're supposed to figure out on your own in your class, and it is tricky because, of course, imperatives usually don't have subjects. So an explanation and your analysis will have to account for that if you need to analyze it as a full sentence, as opposed to, for example, just a verb phrase.