needn't 不需要;不必;强调客观条件 .常做must问句的回答：Must I finish my homework today? No, you needn't.
mustn't不许，不准，不允许 ;命令的口气，语气最强烈。”禁止”You mustn't step on the grass. 甲：Must I finish my homework?
乙：No,you needn'd/don't need to/don't have to
shouldn't 是“（按常理来说）不应该”表义务。意为“应该”（某件事宜于做），用于各种人称。， You shouldn′t waste anytime. 你不应该浪费时间。
oughtn't不应该 表义务，意为“应该”（因责任、义务等该做），口气比should稍重。
You oughtn't to have crossed the road when the lights were red. 你不该在红灯时过马路.
