Email: "Be good" vs. "Be well" [EH]

"Be good" means "Behave well." "Be well" means "Be healthy" (as in Lord of the Rings, "Waes thu hal!" means "Be thou heal[thy]!").

"Good" is an adjective (modifies a noun). "Well" is an adverb (modifies a verb or adjective). So it's OK to say "Be good" so long as you mean "Behave yourself."

By the way, it's OK to say "Do good" when the good is a thing ("He has done good deeds for the poor"). It's not OK to say "Do good!" as in "Do us proud!" unless you are intentionally being colloquial. (I have no problem with someone saying "ain't" or "there's three" so long as he or she knows it's wrong -- and, better, cringes inside when saying or hearing it!)