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must(should,need, ought to, could might)have done用法比较

must(should,need, ought to, could might)have done用法比较
一、在虚拟语气中的用法 1. should have done 表示“过去本应该做某事却未做。”其否定结构 shouldn't have done 表示 “过去本不该做某事却做了。” 2. ought to have done 表示“过去本应该做某事却未做。”其否定结构 oughtn't to have done 表示 “过去本不该做某事却做了。” 3. need have done 表示“过去本有必要做某事却未做。”其否定结构 needn't have done 表示 “过去本没必要做某事却做了。” 4. could (不能用 can) have done 表示“过去本能够做某事却未做。”注意:其否定形式 couldn't have done 没有虚拟语气的用法,couldn't have done 只能表推测,相当于 can't have done,意为:“过去不可能做了某事。” 5. might (不能用 may) have done 表示“过去本可以做某事却未做。” 注意:其否定形式 might not have done 没有虚拟语气的用法,might not have done 只能表推测,相当于 may not have done,意为:“过去可能没有做某事。” 二、表推测的用法 1. must have done 表示对过去某事的肯定猜测,译为:过去肯定做了某事。不存在 mustn't have done 的形式。其否定或疑问形式须用 can(could) 来表示. 例如: Since the road is wet, it must have rained last night. 既然路是湿的,那昨天晚上肯定下雨了。 He can't have missed the way. I drew him a map. 他不可能迷路。我(当时)给他画了张图。 “The dictionary has disappeared. Who could have taken it?” 词典不见了,(过去)谁可能把它拿走了? 2. may / might have done may / might have done 表示“过去可能做了某事”。may 比 might 表示的可能性在说话人看来稍大些。may/might not have done表示“过去可能没有做某事。”例如: I can't find my keys. I may / might have left them at the school yesterday. 我找不到我的钥匙了。我可能昨天把他们落在学校了。 John may/might not have passed the exam; he looks very sad.约翰可能没有通过考试。他看起来很忧伤。 注意:may/might have done表推测不能用于疑问句中。 3. can/could have done 表推测一般用在否定句和疑问句中,表示不相信或怀疑的态度。 Can/Could he have passed the exam?他可能通过了考试吗? I think that he couldn’t/can't have gone abroad. I saw him just now.我认为他不可能出国了。我刚才还看见他了。 注:表示推测过去某动作发生的可能性时,就表示的可能性程度而言,must 最大,could 其次,may 更次之,might 最小。例如: “I wonder how Tom knew about your past.” “我想知道汤姆是怎么知道你的过去的。” “He must / could / may / might have heard of it from Mary.” “他肯定/很可能/可能/兴许(没准儿)已从玛丽那儿听说此事了。”



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