虚拟语气用来表示说话人的主观愿望或假想，而不表示客观存在的事实，所说的是一个条件，不一定是事实，或与事实相反。
1. lf条件句
对现在时态的虚拟从句一般过去时主句过去将来时（would /could/should do)
例句：If i were you ，i would study hard
对过去时态的虚拟从句过去完成时主句would/should /could have done
例句：If you had come five minutes earlier, you would have meet him
对将来时态的虚拟从句if 主语+were to do 主句过去将来时
if 主语+should do
if 主语+did
例句：If it were to snow tomorrow ，what would we do?
2. 接that从句表虚拟，要用should do形式
这些词主要有：insist / advi /suggest/ order/ command / require / request /demond
insist如果翻译成坚持某种动作才用虚拟语气；翻译成坚持某种观点就不用虚拟语气。例句：He insists he is a student.
这个语句表示的是事实，因此在这个语句中不能使用虚拟语气。
suggest意为“建议”才用虚拟语气，意为“暗示”则不用虚拟语气。
3. 表情绪.观点的形容词或名词也要用虚拟语气.
如:necessary、important、impossible、natural、strange、wrong、better、a pity 等。句型：that +从句，从句的谓语动词都要用should+原型或只用动词过去式。
4. 表示“早该做某事了”时，虚拟语气的结构为：It is (high / about) time that + 主语+ 动词的过去式/ should + 动词原形
这些是比较常用的，如果还想知道的话可以再问
虚拟语气
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 狗日批
to work right away becau she thinks that if she were to get a job she probably wouldn't be able to e 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ha李白是剑客吗
d , 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 n ot 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otherwi等表达出来。例如：We didn't know his telephone number; otherwi 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, propo, 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，如果宾语从句销售部经理
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 nur 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 clasd 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 raid the demand that their pay be incread to cope with the inflation.
⒊would （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
⒈it 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, esntial, imperative, urgent, preferable, vital, advisable等，例如It is necessary that you listen to the teacher carefully.或者It is e仔羊计划
sntial 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 th
at 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 himlf.（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.
主句与从句的动作发生在不同的时间，这时主，从句谓语动词的虚拟语气形式因时间不同而不同，这叫做混合条件句。
If you had asked him yesterday, you would know what to do now.
（从句与过去事实相反，主句与现在事实相反。）
If it had rained last night (过去), it would be very cold today (现在).
4.虚拟条件句的倒装
虚拟条件句的从句部分如果含有were, should, 或had, 可将if省略，再把were, should或had 移到从句句首，实行倒装。
Were they here now, they could help us.
=If they were here now, they could help us.
Had you come earlier, you would have met him
=If you had come earlier, you would have met him.
Should it rain, the crops would be saved.
=Were it to rain, the crops would be saved.
注意：
在虚拟语气的从句中，动词'be'的过去时态一律用"were"，不用was，即在从句中be用were代替。
If I were you, I would go to look for him.
如果我是你，就会去找他。
If he were here, everything would be all right.
如果他在这儿，一切都会好的。
典型例题
_____ to do the work, I should do it some other day.
A. If were I
B. I were
C. Were I
D. Was I
答案C. 在虚拟条件状语中如果有were, should, had这三个词，通常将if省略，主语提前, 变成were, should, had +主语的形式。但要注意，在虚拟条件状语从句中，省略连词的倒装形式的句首不能用动词的缩略形式。如我们可说Were I not to do., 而不能说Weren't I to do.
5特殊的虚拟语气词：should
1）It is demanded / 难忘的春节作文300字
necessary / a pity + that…结构中的主语从句的谓语动词要用should 加动词原形，should 可省略。
句型：
(1)suggested
It is (2)important that…+ (should) do
(3) a pity
(1)suggested, ordered, propod, required, demanded, requested, insisted；+ (should) do
(2)important, necessary, natural, strange
a pity, a shame, no wonder
(3)It is suggested that we (should) hold a meeting next week.
It is necessary that he (should) come to our meeting tomorr轻混凝土
ow.
2）在宾语从句中的应用
在表示命令、建议、要求等一类动词后面的从句中。
order, suggest, propo, require, demand, request, insist, command, insist + (should) do
I suggest that we (should) hold a meeting next week.
He insisted that he (should ) be nt there.
注意：如suggest, insist不表示"建议" 或"坚持要某人做某事时"，即它们用于其本意"暗示、表明"、"坚持认为"时，宾语从句用陈述语气。
The guard at gate insisted that everybody obey the rules.
判断改错:
（错）You pale face suggests that you (should) be ill.
(对) Your pale face suggests that you are ill.
(错) I insisted that you ( should) be wrong.
(对) I insisted that you were wrong.
3）在表语从句，同位语从句中的应用
在suggestion, proposal, idea, plan, order, advice等名词后面的表语从句、同位语从句中要用虚拟语气，即（should）+动词原形。
My idea is that we (should) get more people to attend the conference.
I make a proposal that we (should) hold a meeting next week.
6.wish的用法
1）用于wish后面的从句，表示与事实相反的情况，或表示将来不太可能实现的愿望。其宾语从句的动词形式为：
真实状况wish后
从句动作先于主句动词动作过去时过去完成时
（be的过去式为were）
从句动作与主句动作同时发生现在时过去时
（had + 过去分词）
将来不大可能实现的愿望将来时would/could +
动词原形
I wish I were as tall as you. 我希望和你一样高。
He wished he hadn't said that. 他希望他没讲那样的话。
I wish it would rain tomorrow. 我希销售文员
望明天下雨就好了。
2）Wish to do表达法。
Wish sb / sth to do
I wish to e the manager. = I want to e the manager.
I wish the manager to be informed at once. (= I want the manager to be informed at once.)
7.比较if only与only if
only if表示"只有"；if only则表示"如果……就好了"。If only也可用于陈述语气。
I wake up only if the alarm clock rings. 只有闹钟响了，我才会醒。
If only the alarm clock had rung. 当时闹钟响了，就好了。
If only he comes early. 但愿他早点回来。
8.It is (high) time that
It is (high) time that 后面的从句谓语动词要用过去式或用should加动词原形，但should不可省略。
It is time that the children went to bed.
It is high time that the children should go to bed.
9 need "不必做"和"本不该做"
didn't need to do表示：过去不必做某事, 事实上也没做。.
needn't have done表示：过去不必做某事, 但事实上做了。
John went to the station with the car to meet Mary, so she didn't need to walk back home. 约翰开车去车站接玛丽，所以她不必步行回家了。
