虚拟语气是谓语动词的一种特殊形式，用来表示说话人的主观愿望、假想、猜测、建议、可能或空想等非真实情况。所说的是一个条件，不一定是事实，或与事实相反。
1 虚拟语气在条件句中的用法
英语条件句有真实条件句和虚拟条件句两种。真实条件句所表示的假设是有可能发生的，此时主句不用虚拟语气；而虚拟条件句则表示一种假想，与事实相反或不大可能会发生，此时用虚拟语气。
例 If I have time, I will go. 假如我有时间，我就去。（陈述语气）
If I were you, I would go. 假如我是你，我就去。（虚拟语气）
虚拟条件句可分为两类：一类是叙述与现在事实相反的情况, 一类是叙述与过去事实相反的情况。另外，还有一类用于推测将来的情况，也称作推测条件句。其构成列表如下（would 有时可换用should, could或might）：
例 If I knew his telephone number, I would tell you. 如果我知道他的电话号码，我就会告诉你。
If she studied harder, she might pass the exam. 如果她再努力些，就能通过考试了。
② if引导的条件状语从句，与过去事实相反。
例 If she had taken my advice, she would not have made such a mistake. 如果她听我的劝告的话，就不会犯这样的错误了。
If we had got there earlier，I should / would have met her. 如果我们早到那儿，我就会见到她。
③ if引导的条件状语从句，与将来相反。
例 If there were a heavy snow next Saturday，we would go skating. 如果下周六下大雪，我们就去滑冰。
If she were to marry Jim, she would be happy. 如果她嫁给吉姆，她会幸福的。
注： I would / should / could / might主句谓语中的should主要用于第一人称后；would表示结果，还表示过去经常做某事，might表示可能性，could表示能力、允许或可能性。例如：
If you tried again, you would succeed. 要是你再试一试，你就会成功的。 (would表结果)
If you tried again, you might succeed. 要是你再试一试，你可能会成功的。 (might表可能)
If you tried again, you could succeed. 要是你再试一试，你就能成功了。(could表能力)
④ 错综时间虚拟条件句
错综时间虚拟条件句即条件从句与主句所指时间不一致，如从句指过去，而主句即指的是现在或将来，此时应根据具体的语境情况选用适当的虚拟语气形式。
例 If it had rained last night, the ground would be wet now. 要是昨晚下过雨的话，现在地面就会是湿的。
You would be much better now if you had taken my advice. 假若你当时听我的话，你现在就会好多了。
⑤ If虚拟条件句的否定（含蓄条件句）常考两个句型：If it weren’t for ... 和If it hadn’t been for ... ，其意为“若不是（有）”“要不是”。
例 If it weren’t for water, no plant could grow. 要是没有水，植物就无法生长。
If it hadn’t been for your assistance, we wouldn’t have succeeded. 如果没有你的帮助，我们不会成功的。
⑥ 当虚拟条件句中含有were，had，should，could等动词时，可以将if省略，然后将were，had，should，could 等词移至句首。
例 Were they here now, they could help us. (=If they were here now, they could help us.) 他们现在在的话，就会帮助我们了。
Had you come earlier, you would have met him. (=If you had come earlier, you would have met him.) 你来得早一点儿，就碰到他了。
2 其他使用虚拟语气的常见结构或从句
① 动词 wish后的宾语从句用虚拟语气
此句型表示与现在事实相反时，用 were 或动词的过去式；与过去事实相反，用had+过去分词或 would / could+ have +过去分词；表示将来没有把握或不太可能实现的愿望, 用 would / should（could, might） +动词原形。
例 I wish he were (was) here. 他在这儿就好了。（有时也用was）
I wish you would go with us tomorrow. 但愿你明天跟我们一块去。
I wish he had taken my advice. 那时他要是听我的话就好了。
Linda wished she hadn’t done it. 琳达希望她没做那件事。
② if only与wish一样，也用于表示与事实相反的愿望，其后所跟的虚拟语气的时态与wish后所接时态的情况相同。
例 If only he had had more courage! 他再勇敢一些就好了。
If only I had listened to my parents! 我要是当时听了父母的话就好了。
注： if only 通常独立使用，没有主句。
③ 在would rather, would sooner, had better, would as soon, just as soon后的从句中,句子谓语习惯上要用虚拟语气，具体用法为：
a. 一般过去时表示现在或将来的愿望
例 I’d rather you went tomorrow (now). 我宁愿你明天（现在）去。
We’d rather he paid us the money tomorrow. 我们宁愿他明天付给我们那笔钱。
b. 用过去完成时表过去的愿望
例 I’d rather you hadn’t said it. 我真希望你没有这样说过。
I’d rather she hadn’t done that. 我真希望她没这样做过。
注： would rather ... than 和 would ... rather than 后的句子也用虚拟语气，这两个结构中than后的从句要用should +动词原形，而且than后的从句可用也可不用that引导。例如： I would rather die than (that) she should know the secret. 我宁愿死，也不会让她知道那个秘密。
He’d do anything rather than (that) he should live with such a shameless woman.他宁死也不愿同这样一个无耻的女人生活在一起。
④ as if （though）从句用虚拟语气，以as if （as though）引导的方式状语从句或表语从句，有时用虚拟语气，则与wish用法相同。
例 They treat me as though I were a stranger. 他们待我如陌生人。
He talks as if he had been abroad. 他说起话来好像曾经出过国。
注： a. 从句所表示的内容若为事实或可能为事实，也可用陈述语气。如：
It looks as if we’ll be late. 我们似乎要迟到了。
This meat tastes as if it has already gone bad. 这肉吃起来似乎已经坏了。
b. 注意 It isn’t as if ... 的翻译：
It isn’t as if he were poor. 他不像穷人的样子（他又不穷）。
It isn’t as if you were going away for good. 又不是你离开不回来了。
⑤ 在 lest，for fear that（以免），in case（以防）引导的目的状语从句中的虚拟语气。
例 She walked quietly lest she （should) wake up her roommates. 她走得很轻以免吵醒她的室友。(www.xing528.com)
I will not make a noise for fear (that) I should (might) disturb you. 我不会作声的，以免打扰你。
⑥ 表“建议”“命令”“请求”等的宾语从句后需要接虚拟语气，如建议：advise, suggest, propose, recommend；命令order, command；请求ask, demand, require, request；指示direct；督促urge；希望desire；坚持insist；打算intend；安排arrange等。
例 I insisted that he (should) stay. 我坚持要他留下。
She urged that they go to Europe. 她督促他们到欧洲去。
They suggested that we should leave early. 他们建议我们早点动身。
She requires that I (should) appear. 她要求我出场。
He arranged that I should go abroad. 他安排我去国外。
She desires that he do it. 她希望他做此事。
注： suggest表”暗示”，insist表”坚持说”时不用虚拟语气，比较：
He insisted that I had read his letter. 他坚持说我看过他的信。
He insisted that I should read his letter. 他坚持要我看他的信。
He suggested that we (should) stay for dinner. 他建议我们留下吃饭。
I suggested that you had a secret understanding with him. 我觉得你与他心照不宣。
⑦ 在表“建议”“要去”“命令”等名词后的表语从句或同位语从句，也需要用虚拟语气should+动词原形，其中should可以省去。
例 Our suggestion is that you (should) be the first to go. 我们的建议是你应该第一个去。
The demand is that the troops (should) withdraw at once. 按照命令，部队应该马上撤退。
注：诸如此类的单词有advice, agreement, command, decision, decree, demand, determination, indication, insistence, order, preference, proposal, request, requirement, stipulation, suggestion, idea, plan, order等。
⑧ It is (was)+形容词后的that从句中用虚拟语气，从句中用should+动词原形（should可省略）。这类形容词有important, natural, strange, necessary, surprised, appropriate等。
例 It is ordered that the army (should) get there by 4 a.m. 根据命令，军队应该在凌晨四点到那儿。
It is necessary that some immediate effort (should) be made. 必须立即采取行动。
注：类似的形容词还有advisable, anxious, compulsory, crucial, desirable, eager, essential, fitting, imperative, impossible, improper, obligatory, possible, preferable, probable, recommended, urgent, vital等。
3 在定语从句中虚拟语气的运用
句型It is / was (high) time that +完整句子，句子的谓语动词形式用一般过去时或者should do形式且该should不可以省略。（注意与宾语从句中的用法区别开来）
例 It’s time that we went (should go) to school. 我们该去学校了。
It is high time that we went (should go) to bed. 我们该就寝了。
注：若该句型略微调整成It is或was the first time that +完整句子（first也可能是其他序数词)，那这个句子谓语动词不再采用上述形式，而是若主句用的是is，则that后面的句子的谓语动词部分用现在完成时（have / has done）；若主句用的是was，则that后面的句子的谓语动词部分用过去完成时（had done）。
4 -ever词引导的让步状语从句中虚拟语气的运用
在 whatever，whichever，whenever，whoever，wherever，however，no matter wh-等引导的让步状语从句中，从句虚拟语气结构为：指现在或将来， may +do；指过去，may +have done。
例 We will finish it on time no matter what may happen. 不管发生什么事，我们都要按时完成工作。
I will wait for him no matter how late he may come. 不管他来得多么晚，我都会等他。
You mustn’t be proud whatever great progress you may have made. 不管你取得了多么大的进步，你都不能骄傲。
We must respect him no matter what mistakes he may have made. 不管他犯过什么错误，我们必须尊重他。
5 祝愿话语中虚拟语气的运用
表示祝愿话语中也可以用虚拟语气
例 May godbless you. 愿上帝保佑你。
May you enjoy many years of health and happiness. 祝您健康长寿，美满幸福。
6 混合型与含蓄型虚拟语气
(1) 混合型虚拟语气
当虚拟条件从句与结果主语所表达的时间不一致时，被称为“混合条件句”，这种虚拟语气被称为“混合型虚拟语气”，动词的形式要根据它所表达的时间作相应调整。
例 If you had followed the doctor’s advice, he would be all right now. 如果我听医生的建议，我现在已经痊愈了。（条件从句表达的时间是过去，因此用had+过去分词;主句表示的时间是现在，因此用would+动词原形）
(2) 含蓄型虚拟语气
有时候，虚拟条件不是通过if引导的条件句来表示，而是暗含在上下文中。
① 用but for, without（如果没有）等来代替条件从句。
例 Without electricity human life would be quite different. (=If there were no electricity, human life would be quite different.) 没有电的话人类的生活会大不一样。
② 用otherwise, or (or else), even though等表示与上文的情况相反，从而引出虚拟语气。
例 I lost your address. Otherwise, I would have visited you long before. (=I lost your address. If I hadn’t lost your address, I would have visited you long before.) 我弄丢了你的地址。否则的话，我早就拜访你了。
③ 虚拟条件通过but暗示出来。
例 He would have given you more help, but he was too busy. 他本来会给你更多的帮助，但是他太忙了。（也就是说，如果那时他不忙，他可以给你更多的帮助。句中buthewastoobusy实际上暗示了一个虚拟条件——如果那时他不忙。）
He would lose weight, but he eats too much. 他本来可以减肥的，但是他吃得太多了。（也就是说，如果他吃得不多的话，他是可以减肥的。句中的butheeatstoomuch实际上暗示了一个虚拟条件——如果他吃得不多。）
雅思/托福真题解析
1. If it were not for this faculty，they would devour all the food available in short time and would probably starve themselves out of existence.
分析 分句1 是虚拟句的条件从句，分句2、3是并列主句。
分句1: If it were not for this faculty
分句2: they would devour all the food available in short time
分句3: would probably starve themselves out of existence
译文 如果不是具有这种能力，它们就会在短时间内吞掉所有能得到的食物，而且很可能会因饿死而灭绝。
2. Indeed，had it not been for the superb preservation of these fossils，they might well have been classified as dinosaurs.
分析 虚拟语气，条件从句had it not been for the superb preservation of these fossils省略了if，助动词had提前；正常语序为if it had not been for the superb preservation of these fossils。
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