，要根据不同情况，用词不同虚拟语气。这也是虚拟语气不好记的原因。
1.表示与现在事实相反的情况：
从句：If主语+过去时
主句：
主语+should/would/could/might+do
例句:
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）
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. 如果他再努力些，就能通过考试了。（事实：没有努力）
she looked at me as if I had been a stranger. 她看我的样子好像我是一个陌生人。（事实：我并非陌生人）
2.表示与过去事实相反的情况
从句：If主语+had+done
主句：
主语+should/would/could/might+have done
例句:
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）
2. 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.表示对将来情况的主观推测
从句：if+主语+were to do
主句：
主语+should/would/could/might+do
例句:
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）
If there were a heavy snow next Sunday, we would not go skating.
如果下周日下大雪，我们就不能去滑冰了。（事实：不知能否下雪）
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。
指我们平常说的说话人说话的口气。（在英语中，语气除了指语调以外，最主要的是通过动词发生变化而表示不同语气）
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）
二、虚拟语气
1） 虚拟语气概念
虚拟语气用来表示说话人的主观愿望或假想，所说的是一个条件，不一定是事实，或与事实相反。
2） 在条件句中的应用
条件句可分为两类，一类为真实条件句，一类为非真实条件句。非真实条件句表示的是假设的或实际可能性不大的情况，故采用虚拟语气。
三、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
eg If he doesn’t hurry up, he will miss the bus. ( 真实条件状语)
If he was free, he would ask 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. 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表示对将来情况的主观推测（可能相反或可能性很小）
从句
主语例句
If+主语+
① should+动词② did
③were to do
(①通常与一个表示时间状语连用)其 中were to do可能性最小，
should/would/could/might+动词原形If it rained tomorrow our picnic
would be put off.
万一那天下雨，我们的郊游
就推迟。
eg: If he would come here tomorrow, I should/would talk to him. 如果他哪天来这儿的话，我就跟他谈谈。（事实：来的可能性很小）
If there were a heavy snow next Sunday, we would not go skating. 如果下周日下大雪，我们就不能去滑冰了。（事实：不知能否下雪）
If she were to be there 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得始末。
四、虚拟语气的其他用法
①、虚拟语气用在wish 后的宾语从句
a、表示与现在事实相反的愿望，谓语动词用过去式
eg: I wish I had your brains.我希望我有你那样的头脑。(事实：我根本比不上你)
b、表示过去事实相反的愿望，谓语动词：had+v-ed
eg: I wish I had known the truth of the matter.我希望我原来知道这件事的真相。（事实：原来不知道）
c、表示将来难以实现的愿望
谓语动词：should/would + 动词原形
eg: I wish I should have a chance again.我希望我还能有一次这样的机会。（事实：很难再有这样的机会了）
②虚拟语气用在suggest(建议)、insist(坚持)、demand(要求)、order(命令)等动词后的宾语从句中。
在这种用法中，无论主句谓语动词为何种时态，从句的谓语动词都用：“should + 动词原形”或只用“动词原形”。
如 He suggested that we (should) take the teacher’s advice
He insisted that we (should) take the teacher’s advice
He demand that we (should) take the teacher’s advice
He ordered that we (should) take the teacher’s advice