1、对现在假设的情况
从句:If+主语+动词一般过去时(Be动词用were)
主句:主语+ should/would/might/could+do；
If I were you, I would take an umbrella.
如果我是你,我会带把伞。(事实:我不可能是你)
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。(事实:不知道)
If there were no air or water, there would be no living things on the earth.
如果没有水和空气,地球上就不会有生物。(事实:地球上既有空气也有水)
2、对过去没做的事情的假设
实际还是没做
从句：If+主语+had+done
主句：主语+should/would/might/could+have done
If I had got there earlier, I should/would have met her.
如果我早到那儿,我就会见到她。(事实:去晚了)
If he had taken my advice, he would not have made such a mistake.
如果他听我的劝告的话,就不会犯这样的错误了。(事实:没有听我的话)
3、对过去没做的事情的假设
实际还是没做
从句:
①if+主语+were to do
②if+主语+should+do
③if+主语+动词一般过去式(be动词用were)
主句:
主语+should/would/might/could+do
例:
If he should come here tomorrow, I would talk to him.
如果他明天来这儿的话,我就跟他谈谈。(事实:来的可能性很小,不可能)
If there were a heavy snow next Sunday, we would go skating.
如果下周日下大雪,我们就去滑冰。(事实:下雪可能性很小,不可能)
If she were to be here next Monday, I would tell her about the matter.
如果她下周一来这儿的话,我就会告诉她这件事的始末。(事实:来的可能性很小,不可能)
4、错综条件句
有时,虚拟条件句中, 如果主句和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,而主句的动作与正在发生的事实不符。
例:
If I had worked hard at school,I would be an engineer now.
如果我当时在学校学习刻苦的话,我现在就是一个工程师了
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。
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。
例:
If he were free today, we would have sent him to Beijing.
如果他今天有空的话,我们就已经派他去北京了。
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。
扩展资料
应用条件：
在表示假想的虚假的、与事实相反的或难以实现的情况时用虚拟语气，表示主观愿望或某种强烈情感时，也用虚拟语气。即当一个人说话时欲强调其所说的话是基于自己的主观想法，愿望，假想，猜测，怀疑或建议，而不是根据客观实际，就用虚拟语气。主要是英语语法的一种表达。
参考资料来源：百度百科-虚拟语气