虚拟语气的用法
虚拟语气(subjunctive clause)是指所说的话只是一种与事实相反的主观愿望、假设或建议等。虚拟语气可用于条件状语从句(conditional adverbial clause)、主语从句(subject clause)、表语从句(predictive clause)、同位语从句(appositive clause)以及其他结构中。
1. 非真实条件状语从句中的虚拟语气
If 条件状语从句中虚拟语气的形式
在 if 非真实虚拟条件状语从句中，主句和从句的谓语动词主要有下面几种形式：
假设类型 条件从句谓语动词形式 主句谓语动词形式
与现在事实相反 动词过去时(be 用 were） should/could/might/would+do
与过去事实相反 had + done should/could/might/would+have+done
与将来事实可能相反 动词过去时（或should+do或were to+do） should/could/might/would+do
1.1与现在事实相反
在与现在事实相反的非真实条件句中，从句中的谓语动词用过去时或were。主句中的谓语动词用should（第一人称），would（第二、第三人称），但在美国英语中，主句的所有人称都用would。主句中也可以用“could或might+动词原形”
Examples：
If I knew the fact now, I would tell you.
If there were no gravity, we should not be able to walk.
1.2与过去事实相反
在与过去相反的非真实条件句中，从句的谓语动词用“had+过去分词”。主句中的谓语动词用“should（第一人称）或would（第二、三人称）+have+过去分词”。在美国英语中，条件从句中的谓语动词用“would have +过去分词”。
Examples：
1．I wouldn’t have known what these were for if I hadn’t be told.
2．If she had had the money, she would have bought the painting.
1.3与将来事实相反
表示与将来事实相反的非真实条件句，从句中的谓语动词用“过去时或should+动词原形”或“were to+动词原形”。主句中的谓语动词用“should（第一人称）或would（第二、三人称）+动词原形”，主句也可用“could或might+动词原形”。
Examples：
If you were to see Jane，What would you tell her？
This kind of world is hard and dangerous. But if you should succeed, you would become rich.
2. 省略if 的虚拟条件句的用法
在正式文体中，有时可能把虚拟条件句中连词if省去，而将were，had，should等助动词（不包括行为动词）提到主语前面，形成主谓倒装。如果句中没有were，had或should时，既不能省略if，也不能倒装。
Examples：
1. Were I to meet him tomorrow（=If I were to meet him tomorrow), I should ask him about it.
2. Had I had the money last year ( =If I had had the money last year), I would have bought the house.
3. 混合型虚拟条件句
通常情况下，在非真实条件句中的主句和从句所指时间是一致的，主从句中的谓语动词要么都是与现在事实相反，要么都是与过去事实相反等。但有时也可能指不同的时间，这时要根据上下文的意思采用不同德谓语动词形式。
Examples：
1.If I were you, I would have taken
his advice.(从句指现在，主句指过去）
2.If her husband had not been killed
in the war, she would not be helping
others with the housework now.
(从句指过去，主句指现在）
4. 含蓄虚拟条件句
有时假设的情况并不以条件从句表示出来，而是通过一个介词短语或其他方式表示。常用的词或短语有：without，but for，but that，otherwise， or，but 等。
4.1有时虚拟语气可用but for，without，under，with，等介词短语引导。
Examples:
Under such circumstances, I would probably have done the same.
Without your help, we could not have succeeded.
But for electricity, there would be no modern industry.
4.2有时虚拟语气可以由otherwise, but, or来引导。要注意otherwise和or暗含的虚拟句中，前一部分是陈述语气，后一部分是虚拟语气。but 却与之相反。
Examples:
1. I ran all the way to school, otherwise I would have been late.
2. He felt very tried yesterday, or he would have helped you.
3. I ought to have remembered to tell them the decision this morning, but I was so busy that I forgot to do so.
4.3 but that 也可以引导虚拟语气，连词but that（要不是）引导虚拟语气结构，从句部分是陈述语气，主句部分是虚拟语气
Examples:
But that the soldier caught her, the little girl would have fallen into the river.
But that he was short of money at the time, he would have come to help us.
4.4在某些暗含的虚拟条件简单句中，虚拟语气可通过上下文表现出来。谓语动词用should/would be型和should/would have been型来表示。
Examples:
1. Any man in his position would have done like that.
2. You should have returned the book earlier.
5. 虚拟语气与祈使语气、陈述语气的混合使用。
Examples:
If you should change your mind, let us know.
Even if it should get stormy, I will go.