第1个回答 2012-04-01
mustn’t是禁止, 一般用在法律条文,规章制度,长辈对晚辈,上级对下级,语气强烈
You mustn't take this book out of the reading-room.
shouldn’t 是不应该,You shouldn't talk in class.
needn’t是没有必要,You needn't finish the work today.You can do it tomorrow.
couldn't have done否定的猜测,肯定没做过某事
Mary couldn't have gone abroad,for I met her on the street just now.
shouldn't have done不应该做某事却做了某事,
You shouldn't have gone the cinema last night.你昨晚本不该去看电影。(结果作业没做。)
needn’t have done 没有必要做某事却做了某事
比如说一直下雨,汤姆早上起来给花浇水,妈妈会这么说:You needn't have watered the flowers.
其实情态动词要结合具体的语境来理解,三言两语是说不完整的,最好问具体的题目。