①表示与现在事实相反的假设，从句的谓语动词用“动词过去式(be一般用were)”，主句的谓语动词用“should (would, could, might)+动词原形”。例如：
If you were the UN Secretary General, could you stop the wars on the earth?
If he had a chance, he would/should/ might/could choose to study abroad。
②表示与过去事实相反的假设，从句的谓语动词用“had+过去分词”，主句的谓语动词用“should (would, could, might)+have+过去分词”。
①通过with, without, but for和动词不定式短语等表示虚拟的条件
She’d look better with shorter hair. =if she had her hair cut shorter
Without your help, we could not have completed the task ahead of time. =If it had not been for your help…
But for the heavy snow, I could have gone on holiday. =If there had not been the heavy snow…
She would be shocked to hear the news. =If she heard the news…
It would be easier to do it in this way. =If you did it in this way…
②句中含有or, or else, otherwise, but等词语，暗示句子的某一部分(前句或后句)需用虚拟语气。
He was on business in Beijing at that time; otherwise he might have helped us。
Luckily Mary telephoned to inform me of the meeting, or would have been I absent。
I should have attended my friend’s wedding ceremony, but I couldn’t afford the time。
⑴在主语从句中， 在下面的几种情况下，谓语动词通常用“should +动词原形”
①It is+形容词+that…句型。常见的形容词有important, necessary, natural, funny, strange, surprising, astonishing(令人惊讶的)等。
② It is +名词+that…句型。常见的名词有pity, shame, advice, suggestion, proposal(提议，建议)， requirement, request, desire, order等。
③ It is+动词的过去分词+ that…句型。常见的动词有advise, order, propose, request, suggest, demand, require等。
It is important that we (should) have a good knowledge of computer in modern times。
It is necessary that these useful expressions (should) be learnt by heart。
It is a shame that children (should) be forced to beg in the streets in the capital city。
It's suggested that the school (should) organize an outing when spring comes。
It is required that middle-schools students (should) take at least one-hour exercise every day。
⑵在下列动词后的宾语从句中，谓语动词用“should+动词原形”，should可省略。这类动词有：insist(坚持)， urge (催促)， order(命令)， command(命令)， request(请求，要求)， demand(要求)，require(要求，需要)suggest(建议)，advise(建议)，propose(建议) , recommend(建议， 推荐) 等。
The captain insisted that the wounded soldier (should) be sent to hospital at once。
The policeman demanded the thief (should) tell his name and address。
Doctors strongly recommend that fathers (should) be present at their baby's birth。
If Li Ming had spent more time on her studies, she might/would/could/should have been admitted to Tsinghua University last year。
【记忆小窍门】一个“坚持”和”催促”、二个“命令”， 三个“要求”， 四个“建议”。
⑶在含有advice, order, demand, proposal(提议) requirement, suggestion等名词的表语从句、同位语从句中，谓语动词用“should+动词原形”，should可省略。
The governor gave order that the flood (should) be controlled before dark。
My suggestion is that the project (should) be completed by the end of the year。
在一些名词性从句中，谓语动词用“should+动词原形”或“should +have done”，表示惊讶、意志等感情色彩，译为“竟然，居然”。
I am surprised /shocked that you should speak in such a way。
I find it astonishing that he should be so rude to his mum。
It is strange /surprising that she should not have been invited。
I am glad that your story should have won the first prize。
We wish the friendship between the two countries would last forever。
②He always wishes he were a millionaire and owned a big house and a sports car.
③How he wished he hadn’t wasted too much time on playing computer games while at school.
⑸在would rather/would prefer后的宾语从句中
在would rather/would prefer后的宾语从句中，常用过去时表示与现在或将来事实相反的愿望，用过去完成时表示与过去事实相反的愿望。
⑹在would rather/would prefer从句中
①Frankly speaking, I would rather you came tomorrow. The manager isn’t available today。
②I would rather you had gone to the party with me last night. It was really wonderful。
③—Could I smoke here?
—I'd prefer it if you didn’t smoke in front of the children。
3. 在as if/though 后的方式状语从句中，表示某种情况与现在的事实相反， 谓语动词用过去式;表示即将发生的情况“用情态动词+动词原形”;表示某种的情况有过去的事实相反， 谓语动词用过去完成时。
The coach always treats the players as if they were his own children.
She was suffering from a bad cold. Her head felt as if it would burst。
Gary was behaving as though nothing had happened。
4.在in case, in order that目的状语从句中
在in case 引导的目的状语从句中，谓语动词用“should+动词原形”;在so that, in order that 等引导的目的状语从句中，谓语动词除了用“should+动词原形”外，还可用“might/could/would+动词原形”。
In case you should need any help, here’s my number。
In order that training should be effective it must be planned systematically。
I took a taxi so that I would/could/might be in time for the appointment。
③表示一种假设在将来实现的可能性很小或者根本无法实现，从句谓语动词可用三种形式：①动词过去式 ②were to +动词原形 ③should+动词原形，主句谓语动词用：should (would, could, might)+动词原形。
⑵ 省略连词if 的虚拟条件句中
Had he known my address(=If he had known my address)， he would have visited me earlier。
Should I meet Jay Zhou some day(=If I should meet Jay Zhou some day)， I might tell him that I like him.
Were I you(=If you were I)， I would get on well with my teachers and classmates。
5. 在It is (high) time(that)…定语从句中，谓语用过去时或用should+动词原形。should不可省略。
It’s high time that we devoted/should devote our time to preparing for the coming exam。
6. 在if only感叹句中
If only Xiao Hua’s mother were still alive!
If only I had known the answer when my teacher questioned me。
If only I could fly to the moon in Shenzhou Ⅷ spaceship。
Would you be kind enough to show me the way to the post office? 您能否指点我去邮局的路?
Could you let me have your passport? 看看你的护照好吗?
Might I speak to you for a moment? 我能和你谈一下吗?(使用might比may更客气)
May you succeed in the future!
May the friendship between us last long!
If America had not made wars on Iraq, the Iraqi people would be living a peaceful life now。
If I were you, I would have visited the museum yesterday.
If we didn’t have an exam tomorrow, my friends and I could be playing football now。
If you should change your mind, do let me know。
Should anyone call (= If anyone calls)， please tell them that I’m busy。
Can you read that form carefully, if you wouldn’t mind, and then sign it?
I would appreciate it if you call back this afternoon for the doctor's appointment.
—What would you do if it rains tomorrow?
—We have to carry it on, since we’ve got everything ready。
3. 在一些日常会话中，一些简单句通常暗含虚拟的意味。句中的谓语动词用would/could/ should be等形式或should/would/could/might have been等形式。
It couldn’t be better. 那太好了。
That would be very nice. 太好了。
I couldn’t agree with you more. 我非常同意你的意见。
I wouldn’t have dreamed of it. 我做梦也没有想到啊!
How careless I was! I could have done better in the exam. 我太粗心了，我本来可以考得很好的。
Tom hasn’t arrived yet. I should have written down the address for him. 我本应该把地址写下来的。
—How was you interview?
—It couldn’t have been better. Luckily, I found all the answers to questions。
—Shall I give you a ride as you live so far away?
The smile on the boy’s face suggested that he was happy to die for his country。
The man insisted that he had not stolen anything and (should) be set free immediately。
5. 如果as if/though从句描述的是真实的情况，谓语动词则用陈述语气，而不用虚拟语气。
It looks as if it is going to rain。
It sounds as though she has been really ill。
6. 在“had better/would rather, would like/love to, was/were to+ have done sth ”表示过去希望做某事，但事实上未做。例如：
— Do you mind if I open the window?
—I’d rather you didn’t. I feel a bit cold。
7. had thought/hoped/meant/intended/expected/wanted/planned that + 从句 表示主语过去未曾实现的愿望。
We were to have arrived at 9:00 this morning, but I missed the first bus.
I had hoped that I would be of some help to you, but you didn’t ask me。
—Lucy is crying in the corner now. Did you scold her for her carelessness in her homework?
—Yes, but I’d rather not have done it. 我要是不批评她就好了。
1. (2008山东)Thank you for all your hard work last week. I don’t think we ______ it without you。
A. can manage B. could have managed C. could manage D. can have managed
考点解析：句中的介词短语without you表示一个假设的条件，而时间状语last week表明句子陈述的内容与过去的事实相反，因此最佳答案为B。
2. (2008江西)What a pity! Considering his ability and experience, he ____ better。
A. need have done B. must have done
C. can have done D. might have done
考点解析：“What a pity!”暗示他的表现令人遗憾。鉴于他的能力和经验，他本来可能发挥得更好，因此用might have done，最佳答案为D。
3. (2009年郑州市第二次质量检测) —I wish I ________ the meeting。
—But you didn’t。
考点解析：答语But you didn’t暗示对方没有参加会议。因此说话人说“我真希望我参加了那个会议。”在wish后带的宾语从句中 ，如果表示与过去事实相反的愿望应用过去完成时，因此最佳答案为D。
4. (河南省普通高中2009年毕业班教学质量调研考试) —Why didn’t you come to Mike's birthday party yesterday?
—Well, I________, but I forgot it。
A.should have B.must C.should D.must have
考点解析：答语意为“我本来应该去的，但是我忘记了”。should have done 表示“本来应该做某事，而未做”。作为简略，done 被省略，只保留should have，最佳答案为A。
5. (2009年河北保定市高三调研试题)I ____ to go for a walk, but someone called and I couldn’t get way。
A. was planning B. had planned C. planned D. would plan
考点解析：句意为：我原计划去散步，但是有人来访，我无法走开。had planned to do sth表示过去未能实现的打算。最佳答案为B。
6. (2009年河北保定市高三调研试题)I would like _______ you to my birthday party, but I was busy and forgot to call you。
A. to invite B. inviting C. to have invited D. having invited
考点解析：句意为：我本来想让你参加我的生日宴会，可是因为忙，我忘记了“。would like to have done sth。表示“本来想做某事(而未做)”。最佳答案为C。
7. (江西省重点中学2009届高三第一次联考)—Did you visit the famous cultural relics last month?
—No, we _____ visited it, but we spent too much time shopping。
A. could have B. must have C. can have D. ought to have
考点解析：句意为： “我们本来有时间参观(文化古迹)的，可是我们在购物上花费了太多的时间”。 could have done表示“过去本来可以做某事(而未做) ”。最佳答案为A。