"mustn't","can't","needn't"的区别？

mustn't 语气强，”禁止”:You mustn't step on the grass. 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. 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.