"You should not be sad."
Translation:Bạn không nên buồn.
In english, it seems more natural.. "should" is what is being negated, not "sad" but the Vietnamese construction implies the opposite.
I think you confused "đang" and "được".
đang is the present continuous marker.
được has different uses: it could mean the verb to obtain, to get (sthg); it could be translated as to be to form the passive voice; it could also act as a particle that express ability, possibility, similarly to what "có thể" does.