是这样的,needn't do 和 don't need to do 两者有些差别.
就像needn't have done 和 didn't need to do 一样.
从用法上,前者是情态动词,后者是实义动词.
从意义上,前者暗示已经做了,有“本不需要”的意思,后者暗示还没有做,就是纯粹地“不需要”.
如：You needn't do the housework.(说话时你已经做了家务)
You don't need to do the housework.(说话时你尚未做家务