"Must not" vs. "don't have to" is a famous false friends for many languages. That said, what about a phrase like, say, "nobody must know":
Does that mean that a) nobody is allowed to know, or b) that obviously nobody knows, or both? Is that sentence OK, poor style, or to be reworded to make unambiguous? How would I use "must not know", as in "is not allowed to know" together with "nobody"?