wouldshouldcouldhavedone用法和区别.doc

would/could/might/should have done等 用法和区别一、在虚拟语气中的用法（情态动词都是过去时态！）1. should have done：原本应该做某事，(否)shouldnt have done :原本不该做某事2. ought to have done 原本应该做某事，(否) oughtnt to have done :原本不该做某事注意：ought to do 语气更强，常用于长辈对小辈的用语 或某规则上的要求（严格的规则应用 must ）should do 可用语平辈朋友间，较为客气与口语化3. need have done 原有必要做某事 (否) neednt have done 原本没必要做某事注意：做情态动词，need的过去式仍然是need;做实意动词，need的过去式是needed.4. could have done 原本能够做某事 注意： couldnt have done 无虚拟语气的用法， 只表推测，相当于 cant have done：过去不可能做了某事5. might have done 原本可以做某事 注意： might not have done 无虚拟语气的用法， 只表推测，相当于 may not have done：过去可能没有做某事6.would have done原本会做某事1. I would have told you all about the boys story, but you didnt ask me. 二、对过去事情表推测的用法(情态动词用原型或过去式都可以，只是在说话者看来，可能性大小稍微有所不同而已)1. must have done：过去肯定做了某事。不存在 mustnt have done 的形式。其否定或疑问形式须用 can(could)have done：“不可能做过某事” 来表示： 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 cant/couldnt 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/can 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：过去可能做了某事，may 比 might 表示可能性稍大些。may/might not have done过去可能没有做某事： I cant 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,疑问句中表推测，一般用can/could have done sth.很可能做过某事3. can/could have done 常用于(否定句)和(疑问句),间或(肯定句)Can/Could he have passed the exam ? 他可能通过了考试吗 ？I think that he couldnt/cant 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： “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。”4. would have done:想必会/肯定会做某事President Obama has canceled plans to set stronger air quality rules in the United States.The president said these plans would have forced（想必会/肯定会迫使） businesses to take action at a time when the countrys economy is weak. On Friday, president Obama ordered environment officials to halt a plan that would have affected（想必会/肯定会影响） companies, states and cities. All three would have required