need/must

My try is:
1. Need I finish the assignment by noon? Yes, you must.
2. Need I finish the assignment by noon? No, you needn't.
3. Must I finish the assignment by noon? Yes, you must.
4. Must I finish the assignment by noon? No, you needn't.

paco

in 1 and 3 both 'need to' and 'must' sound fine to me.

in 2 and 4, 'needn't' and 'mustn't' both sound fine to me, but have different meanings. the scope of negation with 'must' is different to the scope of negation with 'need'. compare,

you needn't X = 'it is not required that you X'
you mustn't X = 'it is required that you do not X'.

sam

Sam Cin 2 and 4, 'needn't' and 'mustn't' both sound fine to me

Dialigue A
- Is it required that I finish my work by noon?
- No, it is not required that you finish your work by noon.
Dialigue B
- Is it required that I finish my work by noon?
- No, it is required that you do not finish your work by noon.

To me

Paco2004
Dialigue B
- Is it required that I finish my work by noon?
- No, it is required that you do not finish your work by noon.

To me the dialogue B sounds weird.

yes, it would sound off to me too in certain contexts, as would the other examples, though all of them are conceivable. here it's out of context - a

Hi Sam

Thanks for the reply. I agree that the use of "mustn't" is reasonable in such a context as you told.

paco