wouldshouldcouldhavedone用法和区别

1、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）shouldn't have done : 原本不该做某事2. ought to have done原本应该做某事，（否）oughtn't to havedone :原本不该做某事注息：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

2、事（ 否）needn't 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：couldn't have done无虚拟语气的用法，只表推测，相当于can't 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

3、. I would have told you all about the boy's story, but you didn't 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 mustn't 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

4、晚上肯定下雨了。He can't/couldn'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/can have takenit? ”词典不见了，（过去）谁可能把它拿走了？2. may / might have donemay / 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 can't find

5、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 ）

6、 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：“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

7、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 qualit y rules in the United States.The president said these plans wo uld have forced （想必会/肯定会迫使）businesses to take action at a time when the country's econo my 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 requi