Are they the same? Is there a difference?
We can use both forms interchangeably, as in:
- You don't need to water the garden this evening. It's going to rain tonight.
- You needn't water the garden this evening. It's going to rain tonight.
- There's no need to water the garden this evening. It's going to rain tonight.
Sometimes, however, there can be a slight difference in meaning.
- No, you needn't / don't need to pay for this phone call. It's on the house. (real time situation)
- In most countries you don't need to pay for emergency calls. (a habitual activity, needn't impossible here: In most countries you
needn'tpay for...)
Negative equivalents of 'must'
Needn't is also one of the negative equivalents of must:
- You must wear your uniform every day, except Sunday. On Sunday you needn't wear it.
[source: Practical English Usage & A-Z English Grammar]
