"You must not eat so fast!"
Translation:Nesmíš jíst tak rychle!
This sentence totally mystifies me. In English this is a very common expression. But the word “mustn’t / must not” while eating is almost always used as a strong suggestion — meaning “shouldn’t / should not”. “You shouldn’t eat so fast.” My understanding of “nesmíš” means “not allowed”, as if it’s some sort of rule. Would “Nesmíš jíst tak rychle” be typically used as a strong suggestion, and if not, (1) In what situation would this Czech phrase be used; and (2) How would you say “You shouldn’t eat so fast”?
The father of one of my friends worked in a lumber camp as a boy. He claimed that the cook would hit the lumberjacks with a stick if they ate too slowly -- eating was part of their job, and they should not dawdle while doing it. Their lives belonged to the company for the duration of the job. In that camp, 'You must not eat so slowly' had the meaning you suggest.
But the common English expression, 'you must not eat so fast' is often followed by the caution, 'or you will make yourself sick.' (It is often used by self-appointed authorities.) I think that in this expression, 'must' is used to express a strong degree of certainty, as in 'you must be thirsty', which is not to say that you are required to be thirsty. 'You are all wet. You must not have brought your umbrella' is a not-very-parallel example of a clear certainty usage.
In the negative case "you must not" and " you are not allowed to" mean a very similar thing.
In the positive case "you are allowed" and "you may" are very similar.
In Czech "muset" does not do this kind of weird switching of the meaning the English must does:
I must - musím
I must not - nesmím
I have to - musím
I do not have to - nemusím
See how "have to" negates differently from "must". Czech muset is like have to and unlike muset.