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)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 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) 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. 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 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 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,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 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 country's 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 to reduce the amount of chemicals that case smog.
Failure to do so, would have led to（肯定会/想必会导致） federal action.