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,youneedn' stn'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't/don't need to/don't have to shouldn't是“(按常理来说)不应该”,强调建议,suggestion
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