学位英语考试中重点语法知识主要包括：时态、语态、情态动词、虚拟语气、非谓语动词、从句、倒装句、强调句、主谓一致等。今天主要学习有关情态动词的语法知识点。情态动词没有人称的变化。除了ought后接to do，其它的情态动词一般和动词原形一起构成谓语。

       1．must＋现在完成时结构：这一结构表示对过去一个动作比较有把握的猜测。

       例：I believe he _____ an accident, otherwise he would have arrived on time.

       A. would have had B. could have had

       C. should have had D. must have had

       答案为D。

       2．情态动词无人称变化，在句子中和动词原形一起组成谓语。只有ought后面接to do。

       情态动词后接完成时的用法:情态动词用来表示能力、允许、许诺、可能、必须、劝告、意愿等概念或态度。

       3．could＋现在完成时，表示本来能做的事情而没有做。

       例：Mary _____ that coat, but she chose to lend the money to a needy neighbour.

       A. could have bought B. must have bought

       C. can buy D. could buy

       答案为A。玛丽本来可以买那件大衣的，但是她把钱借给一个更急需的邻居了。

       4．should (ought to )have done用来表示本来应该做却没有做的事情。

       例：I’m sorry I couldn’t get in touch with him before he left, I _____ him earlier.

       A. had a telephone B. have phoned

       C. should have phoned D. should be phoned

       答案是C。中文意思是“我很抱歉我没能在他走之前和他联系上，我本应早点儿给他打电话的。”


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