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问句的回答：May I sit here? No, you can't.;
shoudn't语气较委婉，”不应该”
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