would you tell me the difference between need and neednīt, please?
You can say sentences with 'need' in three ways - two of which are negative:
You need to come.
You don't need to come.
You needn't come.
The two negative ones - 'You don't need to come' and 'You needn't come' - are identical in meaning. The form using 'needn't' is used much less.