> "Be not sad" is perfectly legitimate English, but it's archaic and sounds lofty and Shakespearean to modern ears. "Take not the low road." "Spurn not the compassion of others." A lot of costume dramas have dialogue written in this manner -- often for the worse, because the composition of the dialogue distracts from the meaning of it, but certainly a lot of classic English literature is packed with these kinds of constructs.

But, you see, in these other examples, the "not" goes with the verb - "Spurn not the compassion of others", for example, consists of "Spurn not" and "the compassion of others". Whereas I think what Joona was getting at here was that in "Be not sad", it's split up as "be" and "not sad", rather than "be not" and "sad". Certainly, the first form makes sense in this context, whereas with the "Spurn not..." example, it doesn't.