虚拟语气(Subjunctive Mood)这一语法项目是各类英语考试中心测试的重点之一。虚拟语气是一种特殊的动词形式，用来表示说话人所说的话并不是事实，而是一种假设、愿望、怀疑或推测。
Ⅰ用以表示虚拟条件的虚拟语气
⒈ 用if条件从句表示的虚拟条件，是虚拟条件最普通的方式。
① 虚拟现在时表示与现在事实相反的假设，其if 从句的谓语形式用动词的过去式(be 一般用were)，主句用would/ should/ could/ might +动词原形，例如：If I were in your position I would marry her.
② 虚拟过去时是表示与过去事实相反的假设，if 从句的谓语形式用过去完成时即had+过去分词，主句用would / should / could / might + have +过去分词，例如：If it had not rained so hard yesterday we could have played tennis.
③ 大多数的虚拟条件句属于上面三种情况的一种，但并不排除存在条件和后果中，一个和现在情况相反，另一个和过去情况相反，例如：If you had followed what the doctor said, you would not have been so painful now.这个句子在高中出现频率颇高。
④ 但是，如果后果用了虚拟语气，而条件却用陈述语气，这种用法是错的。
⒉ 除了表示虚假条件外，if从句还可以表示对将来的推测，由于是将来还没有发生的，所以谈不上是真实的还是虚假的，只能说这个事情发生的可能性有多大。一般情况下，可以用陈述语气的if从句来表示对一个未来事实的推测，这个事实是完全可能发生的。If从句的谓语形式用一般过去式或用were to / should +动词原形，主句用would / should /could/ might +动词原形，例如：Jean doesn't want to work right away because she thinks that if she were to get a job she probably wouldn't be able to see her friends very often.（1996年1月四级第44题）
⒊ 有时可以把含有助动词、情态动词、be或have的虚拟条件句中的连词if 省去，而将had ， should, were 等词提到主语之前，即用倒装结构，这时候，如果出现not等否定词，否定词需放在主语后面。这种结构在口语中很少使用，但频频出现在各类考试中出现，例如：If it had not rained so hard yesterday we could have played tennis.→Had it not rained so hard yesterday, we could have played tennis.
⒋ 大多数的虚拟条件通过上面所讲的两种方法表达，但在个别句子中也可以通过介词without和介词短语but for表达，副词otherwise等表达出来。例如：We didn't know his telephone number; otherwise we would have telephoned him.（1995年6月四级第46题）值得注意的是，包含but for的句子，谓语动词必定要用虚拟形式，但包含without等短语的句子，谓语动词未必一定要用虚拟语气。
⒌ 有时虚拟条件不是明确地表达出来，而在蕴含在用but引导的从句里，于是便出现了有谓语动词是虚拟语气的主句加上谓语动词是陈述语气的but从句构成的并列复合句，例如I would have hung you but the telephone was out of order.在这样的句子里，如果主句没有用虚拟语气，或者从句用了虚拟语气，都是错的。
Ⅱ用在宾语从句中的虚拟语气
⒈ 在表示愿望的动词wish后的宾语从句中，需用虚拟语气。（wish后的that 常省略），根据主句时态，从句谓语时态相应退后一位，例如：How she wished his family could go with him.
⒉ 在具有愿望、请求、建议、命令等主观意愿的动词（desire, demand, advice, insist, require, suggest, propose, order, recommend, decide …） 后的宾语从句中需用虚拟语气。谓语动词用(should) +动词原形。值得注意的是，如果宾语从句的动词是否定的，否定词not的位置应在动词之前，而不是动词之后。例如：The head nurse insisted that the patient not be move.另外，如suggest表达“暗示”，insist表示“坚持某种说法”时，后面的从句不用虚拟语气，例如：Her pale face suggests that she is ill.或He insisted that he did not kill the boy.
除此之外，上述动词也要求用虚拟式
① 在It is+上述动词的过去分词，其后所跟的主语从句中，如It is suggested that pupils wear school uniforms.
② 在上述动词相应的名词形式作主语+连系动词，其后的表语从句中，例如His suggestion was that classed be re-scheduled.
③ 在对上述动词相应的名词进行解释的同位语从句中，如：The workers raised the demand that their pay be increased to cope with the inflation.
⒊ would (had)rather ， would sooner也用来表达主观愿望，它们之后的宾语从句中需用虚拟语气。谓语动词用过去式表示现在或将来，用过去完成式表示与过去事实相反，例如：I would rather he went right now.
Ⅲ其他形式的虚拟语气
⒈ it is +necessary等形容词后，that主语从句中虚拟形式使用，这类形容词包括necessary, important, essential, imperative, urgent, preferable, vital, advisable等，例如It is necessary that you listen to the teacher carefully.或者It is essential that you be able to pronounce every single word correctly.
⒉ 在It's (high/about) time 之后的定语从句中需用虚拟语气。谓语动词用过去式，例如It is high time that you went to school
⒊ 虚拟语气在as if / as though 引导的方式状语从句中的应用（谓语动词形式与wish后的宾语从句基本相同）表示与现在事实相反或对现在情况有怀疑，谓语动词用过去式。例如：He felt as if he alone were responsible for what had happened. 表示过去想象中的动作或情况，谓语动词用过去完成式。
⒋ 在lest 引导的状语从句中，谓语动词多用虚拟语气，（should ）+ 动词原形。例如： The mad man was put in the soft-padded cell lest he injure himself.（1998年1月四级第38题）
⒌ 在if only 引起的感叹句中需用虚拟语气。谓语动词用过去式或过去完成式。例如： Look at the terrible situation I am in! if only I had followed your advice.
一、过去式：
if 从句：had done
英语语法：虚拟语气的时态
主句：情态动词(would，could，might，should)+have done
英语语法：虚拟语气的时态
二、现在式：
if 从句：did/were
英语语法：虚拟语气的时态
主句：情态动词(would，could，might，should)+do
英语语法：虚拟语气的时态
三、将来时：
if 从句：①should/shall do
②were to do
③did/were
英语语法：虚拟语气的时态
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主句：情态动词(would，could，might，should)+do
虚拟语气是一种特殊的动词形式，用来表示说话人所说的话并不是事实，而是一种假设、愿望、怀疑或推测。判断是真实条件句还是非真实条件句。只有在非真实条件句中才使用虚拟语气。通过句子意思，看假设的条件是否能够实现，能够实现是真实条件句，不能使用虚拟语气；假设的条件不能实现则是非真实条件句，要用虚拟语气。判断这个假设是与哪个事实相反。
通常有三种情况：①与过去事实相反。②与现在事实相反。③与将来事实可能相反。
I: 最基本的虚拟语气句型:
1、 虚拟现在时表示与现在事实相反的假设，其if 从句的谓语形式用动词的过去式(be 一般用were)，主句用would/ should/ could/ might +动词原形。If places were alike, there would be little need for geographers.如果各个地方都一样，就不需要地理学家了。
2、 虚拟过去时是表示与过去事实相反的假设，if 从句的谓语形式用过去完成时即had+过去分词，主句用would / should / could / might + have +过去分词。If he had known your address yesterday,he would have telephoned you.
3、 虚拟将来时是表示对将来实现的可能性很小的或不确定的假设。If从句的谓语形式用一般过去式或用were to / should +动词原形，主句用would / should/could/ might +动词原形。If he were to leave today, he would get there by Friday
4、 省略if 采用倒装语序的条件句。有时可以把含有助动词、情态动词、be或have的虚拟条件句中的连词if省去，而将had ， should, were 等词提到主语之前，即用倒装结构。If he had worked harder, he would have got through the exams.==>Had he worked harder, he would have got through the exams.If he were to leave today, he would get there by Friday.==>Were he to leave today, he would get there by Friday.If I were in your place, I wouldn’t do that.==>Were I in your place, I wouldn’t do that.
5、 有时虚拟条件句并没用if 从句表示出来，而是用介词短语(otherwise, or, without, but for)、上下文或其它方式来表示。如：We didn't know his telephone number; otherwise we would have telephoned him. Without you help, I wouldn’t have achieved so much. But for （“要不是。”的意思）your help, I would not have succeeded.
II: 虚拟语气在从句中用should的情况:
1、 在表示建议，要求，命令，想法的动词后的从句里， 用should + 动词原形， should 可以省略。 如 advise, agree, command, decide, demand, determine, grant, indicate, insist, order, prefer, propose, request, require, stipulate, suggest, urge, vote.常考到的是: suggest, advise, demand, require, propose, insist, order, request.
His doctor suggested that he (should) take short leave of absence.
The author proposed that TV (should) be turned off at least one hour every day.
2、 在表示建议，要求，命令，想法的名词后的从句里， 用should + 动词原形， should可以省略。 如 advice, decision,agreement, command, decree, demand, determination, indication, insistence, order, preference, proposal, request, requirement, stipulation etc.
It was Bill’s suggestion that everyone (should) have a map.
His suggestion was that everyone (should) have a map.
He gave us a suggestion that everyone (should) have a map.
3、 在It is/was +形容词后的that 从句中用should的结构， should 可以省略。 这类形容词常见的有: advisable, anxious, compulsory, crucial, desirable, eager, essential, fitting, imperative（绝对必要）， impossible, improper, important, natural, necessary, obligatory, possible, preferable, probable, recommended, urgent, vital etc.
It‘s natural that she (should) do so.
It is essential that we (should) tell her the news.
4、 在 lest 和 for fear that（以免），in case（以防）从句中用should, should 可以省略。
She walked quietly lest she (should) wake up her roommates.
Have your gun ready in case we should need it.
III: wish 后的 that 从句中:
1、 表示现在或将来的愿望，从句中过去式。
I wish I knew his address.
I wish I were young.
2、 表示过去没有实现或不可能实现的愿望， 从句用过去完成式或would, could, might + have + 过去分词。
I wish you had written to him.
I wish I could have slept longer this morning, but I had to get up and come to class.
3、 如果将wish改成wished, 其后that 从句中动词的形式不变。
4、 如果that 从句中用would ， 一般表示 对现状不满或希望未来有所改变或请求
I wish he would answer my letter.
I wish prices would come down.
I wish you would help me.
I wish you would stop asking silly questions.
IV: 在 It’s about/ high/ first(second, third etc)time 后的that 从句中，用过去式。表示“该是做什么的时候了”
It is about time you were in bed.
It is high time we left.
It is the first time I came here.
V: 在would rather, would sooner, would just as soon 后的that从句中，用过去式或过去完成式，表示“宁愿做什么”
I would rather he came tomorrow than today.
John would rather that she had not gone to the party yesterday evening.
VI: 在if only（“如果。就好了”的意思） 感叹句中，谓语动词与wish宾语从句的虚拟形式相同。
If only he didn’t drive so fast!（现在）
If only she had asked someone’s advice.（过去）
If only the rain would stop.（将来）
VII: 在as if / as though 从句中，表示和现在事实相反或对现在情况有所怀疑，用过去式；表示过去想象中的动作或情况， 用过去完成式。
He speaks as if he were on the spot.
She spoke to me as if I were deaf.
This device operated as though it had been repaired.
注: 1. 在 as if / as though 句中， 如果有可能成为事实， 用陈述语气。
He looks as if he is going to be ill.
2、 在 insist 后的从句中， 如果是坚持自己， 用陈述语气， 坚持别人做什么事情， 用虚拟语气。
She insists that she is right.
She insisted that I should finish the work at once.