may not 语气较弱,”可以不”,You may not finish your homework;
can't ”不能”; You can't be too serious;问句回答时：Can I go out to play?No,you can't.或做may问句的回答：May I sit here?No,you can't.;
couldn't 语气较委婉,”不能”,常做could疑问句的回答：Could I sit here?I'm afraide you couldn't;
needn't 表示不必这么做：常做must问句的回答：Must I finish my homework today?No,you needn't.
mustn't 语气强,”禁止”:You mustn't step on the grass.