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语气是动词的一种形式，表示说话人的看法和态度。英语有三种语气，即陈述语气、祈使语气和虚拟语气。陈述语气主要用于陈述句和疑问句，用于陈述事实、提出看法或问题等;祈使语气用于祈使句，表示请求、命令或警告等;虚拟语气则表示假想、推测或主观愿望，而非客观存在的事实。如：Don’t be late next time(祈使语气)。If I were you, I would not go. (虚拟语气)I wish I had a lot of money. (虚拟语气)。
在虚拟语气中，含有非真实条件句的复合句使用最为普遍，学习时要特别注意从句和主句谓语动词(因动作发生时间不同而具有)的几种不同组合形式。另外，一些特定的英语句型或表达中常需要使用虚拟语气，也应是我们在学习时特别关注的地方。
考点一：虚拟语气的三种基本类型
1.与现在事实相反：①条件从句谓语：动词过去式(be 一般用were)
②主句谓语：would/should/could/might +动词原形
例如：If I saw him now, I would be very happy.
If I were here now, we could ask him about it.
2.与过去事实相反：①条件从句谓语：had+过去分词
②主句谓语：would/should/could/might + have+过去分词
例如：If I had seen him then, I would have been very happy.
If I had been here then, we could have asked him about it.
3.与将来事实相反：(1)条件从句谓语：①动词过去式②should+动词原形③were to+动词原形
(2)主句谓语：would/should/could/might+动词原形
例如：If I were/should be/were to be you tomorrow, I shouldn’t quarrel with him.
Even if 引导的让步状语从句，其谓语形式与if 虚拟条件句相同。
Nothing could have saved the wounded man even if he had been treated without delay.
注意：
1.虚拟条件句中谓语动词变化助记口诀：碰到虚拟条件句，时态向前移一位。假设现在某情况，从句应用过去时。假设过去某情况，应用过去完成时。
2.上述主句谓语中，would表示结果，might表示可能性，could 表示能力、允许或可能性，should主要用于第一人称后。
3.条件从句的谓语动词为be时，不管其主语为单数还是复数通常都用were。
4.与将来事实相反的虚拟语气中，当条件从句谓语动词使用should +动词原形时(表示可能性极小，常译为万一)，主句谓语动词除了would/should/could/might +动词原形这样的虚拟语气外，也可用陈述语气或祈使语气
例如：If I should see him, I will tell him.
If it should rain tomorrow, don’t except me.
考点二：(时间)混合虚拟语气
条件从句的动作和主句的动作发生的时间不一致(如一个是过去发生的，一个是现在发生的)。这时主从句动词的形式要根据时间来调整，使用各自的谓语动词。
例如： Tom would be alive today if the doctor had come sooner last night.
If he had followed th doctor’s advice then, he would be quite all right now.
考点三： 倒装的虚拟条件句(were, had, should)
If 条件句中有were, had(助动词)和should时，可使用倒装结构，但if必须省略。
If I had seen him then, I would have been very happy. =Had I see him then, I would have been very happy.
If I should have time, I would go with you. =should I have time, I would go with you.
若条件从句为否定句，否定词not应置于主语之后，而不能与were, should, had 等缩成weren’t, shouldn’t, hadn’t而置于句首。
If I were not so busy, I would go with you. =Were I not so busy, I would go with you.
考点四：含蓄条件句的虚拟语气(without，with，but，for，or，otherwise或上下文)
用without(=but for)，or (else)，otherwise暗示if从句
例如：but for his help, we wouldn’t have made such great progress.
He must have missed the train, or he would have arrived here then.
当上下文或上下句中的语境暗示的时候，省略条件句，避免重复。
It was not a matter of money. Even a very poor man would have done the same, I think.
They helped me a lot. I don’t know what I would have done without their help.
考点五：虚拟条件句两个常用的固定句式(if it weren’t for…和if it hadn’t been for…)
If it weren’t for…和if it hadn’t been for…是两个很正常的虚拟语气句型，其意为若不是(有);要不是有。
其倒装形式是：Were it not for…和Had it not been for…
此句型可与but for, without “要不是’(后加名词，句子谓语动词常与虚拟语气)等替换。
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