Could somebody please explain, why "We do not have to wait for her today" is not accepted. I do not see much of a difference between this sentence and the proposal "We are not supposed to wait for her today".
have to is basically the same as must, so a bit different meaning from be supposed to.
'we are not expected to wait for her today' is not accepted. I was just experimenting. It seems to me we are in the realm of social expectations, even ethics. In another language 'be expected' might be used. 'Not supposed' can mean 'forbidden'. But in English 'to be supposed' isn't always used as a social expectation, rather as a possibility, eg 'What was I supposed to do?'.
"We are not supposed to" gives the impression that someone told us not to. Is this the same impression that you get from "nemam"?
From my earlier comment: The construction mít-plus-infinitive generally translates as "to be supposed to" or "to be meant to." But since your question is a little nuanced, one of the Czech natives on the team may have something to add.
We should not.... would be Neměli bychom na ni .....
the sentence here is 'we are not supposed to', and that can translate to 'we are not to wait...'
No - "we are not to wait for her" is not acceptable in normal English (sixty years ago in an aristocratic milieu, maybe, but certainly not in any other context). "We are not supposed to wait for her" is ok. What's more, the difference between this sentence and "we shouldn't wait..." is almost non-existent. Finally, none of these three sentences is a natural utterance; you'd be much more likely to say "we're not waiting for her..." or "we don't have to wait for her..."
I'm sorry, but I disagree. "We are not to wait for her" is completely acceptable in the English I'm very familiar with: American English, at least from the Eastern part of the United States.
It sounds fine to me, too, and it implies some external instruction not to wait, whereas 'we shouldn't wait' could be entirely for our own reasons.
My translation “ we needn’t wait for her today “ wasn’t accepted. I can’t think why.
The construction mít-plus-infinitive generally translates as "to be supposed to" or "to be meant to." They have a somewhat different meaning than "to need to." Translations using "need" are not accepted, because that is closer to the verb potřebovat.