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)。
例如：If I saw him now, I would be very happy.
If I were here now, we could ask him about it.
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.
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.
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.
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.
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.
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。