第一大类：虚拟条件句

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。
如：If I have time, I will go.
假若我有时间，我就去。 （陈述语气）
If I were you, I would go.
假若我是你，我就去。 （虚拟语气 ）
第一讲复习
与现在事实相反：从句：If 主语+过去时（Be动词用were）主句：主语+should/would/could/might+do
与过去事实相反：从句：If 主语+had+done；主句：主语+should/would/could/might+have done
对将来的主观推测：从句：①if+主语+were to do 主句：①主语+should/would/could/might+do；从句：②if+主语+did/were 主句：②主语+should/would/could/might+do；从句：③if+主语+should+do 主句：③主语+should/would/could/might+do

第二讲 虚拟语气在条件句中的用法全套解析

第一类：穿越，即错综条件句

有时，虚拟条件句中，结果主句和条件从句的谓语动作若不是同时发生时，虚拟语气的形式应作相应的调整。这种条件句叫错综条件句。
① 从句的动作与过去事实相反，而主句的动作与现在或现在正在发生的事实不符。
If I had worked hard at school, I would be an engineer, too.
如果我在学校学习刻苦的话，我现在也会成为一个工程师了。
If they had informed us, we would not come here now.
如果他们通知过我们的话，我们现在就不会来这里了。
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。
②从句的动作与现在事实相反，而主句的动作与过去事实不符。
If he were free today, we would have sent him to Beijing.
如果他今天有空的话，我们会已经派他去北京了。
If he knew her,he would have greeted her.
要是他认识她的话，他肯定会去问候她了。

第二类：隐身，即省略条件句

1. 省略主句或从句：有时，虚拟条件句中，主、从句可以省略其中一个，来表示说话人的一种强烈的感情。
①省略从句
He would have finished it.
他本该完成了。（主句可能是：If he had started it earlier 如果他早些开始）
You could have passed this exam.
你本能通过这次考试的。(主句可能是：If you had studied harder 如果你再努力一点）
②省略主句
If I were at home now.
要是我现在在家里该多好啊。
If only I had got it.
要是只有我得到它了该多好啊。
2. 省略 if虚拟条件：有时可将条件从句的连词if省略，但此时应用倒装句型，即把从句中的were, should, had 等提到句首。例句：
Were I Tom, I would refuse.
如果我是汤姆，我会拒绝。（标准原句：If I were Tom, I would refuse.）
Should it be necessary, I would go.
假若有必要，我会去的。（标准原句：If it were necessary, I would go.）
Had it not been for the bad weather we would have arrived on time.
若不是天气坏，我们就准时到达了。（标准原句：If it had not been for the bad weather we would have arrived on time.）
3. 省略含蓄条件句：在特定的上下文或一目了然的情况下，if条件句可以省略，或使用介词短语、副词或非谓语动词来代替if条件句。这种虚拟语气表达形式叫做含蓄条件句
①条件暗含在短语中
We didn't know his telephone number; otherwise we would have telephoned him.
我们不知道他的电话号码，否则我们就会给他打电话。（暗含条件是副词otherwise）
Without your help, we wouldn't have achieved so much.
没有你的帮助，我们不可能取得这么大的成绩。（暗含条件是介词短语without your help）
But for your help, I would not have succeeded in the experiment.
如果没有你的帮助，我的实验就不会成功。（暗含条件是but for your help）
②条件暗含在上下文中
I would not have done it that way.
我是不会那么做的。（可能暗含if I were you）
I was busy that day. Otherwise I would have come to help you.
我那天很忙。否则我会来帮你的。（可能暗含if I hadn't been so busy.）
You might come to join us in the discussion.
你可以参加我们的讨论。（可能暗含if you wanted to）
I would have bought the DVD player.
我是会买下那台影碟机的。（可能暗含if I had the money）
But for the storm, we should have arrived earlier.
要不是碰到暴雨，我们还会早些到。（可能暗含if it had not been for the storm）