mustn't和needn't和can't和shoudn't的区别

mustn't 语气强，”禁止”;千万不要的意思 例如:You mustn't step on the grass. 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. 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