英语中的语气分为三类：

　　陈述语气(用于陈述句、疑问句、感叹句)

　　祈使语气(用于祈使句)

　　虚拟语气(用于条件状语从句、宾语从句等)

　　一、虚拟语气

　　如果所说的不是事实，而是一种假设、愿望、建议或是一种实现不了的空想，就用虚拟语气。

　　二、虚拟语气在非真实条件状语从句中的用法

　　1、真实条件状语从句与非真实条件状语从句

　　If he doesn't hurry up, he will miss the bus.( 真实条件状语)

　　If he was free, he asked me to tell stories.(真实条件状语)

　　If I were you, I would go at once.(非真实条件状语从句)

　　If there was no air, people would die.(非真实条件状语从句)

　　2、虚拟语气在非真实条件状语从句中的用法及动词形式

　　① 表示与现在事实相反的情况

　　(条件)从句谓语动词形式主句谓语动词形式

　　谓语动词用过去式(be用were) should/would/could/might+动词原形

　　eg：If I were you, I'd 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)

　　If I had any money with me, I could lend you some.

　　如果我带钱了，我就会借给你些。(事实：没带钱)

　　If he studied harder, he might pass the exam.如果他在努力些，就能通过考试了。(事实：学习不用功)

　　②表示与过去事实相反的情况

　　从句谓语动词形式 主句谓语动词形式

　　Had+过去分词 Should/would/could/might+have+过去分词

　　eg If I had got there earlier, I should/could 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.