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或实际可能性不大的情况，故采用虚拟语气。例如：
if he doesn't hurry up,he will m**s the bus.如果他不快点，他将错过巴士。(真实)
if he ** free,he will ask me to tell stories.如果他是空闲的，他会要求我讲故事。（真实）
if i were you,i would go at once.如果我是你，我马上就会去。（我不可能是你。非真实，虚拟语气，与事实相反）
if there were no air,people would die.如果没有空气，人就会死亡。（不可能没有空气。非真实，虚拟语气，与事实相反）
1、表示与事实相反的情况
从句：if+主语+过去时(be动词用were)
主句：主语+should/would/might/could+do：
1．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）
2．if i knew h** 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）
3．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)
4．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：没有带钱）
5．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）
6．if i were you,i'd wear a shirt and tie.
如果我是你，我就会穿衬衫和领带。(事实：我并不是你）[1]
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/might/could+h**e done
eg：
1．if i had got there earlier,i should/would h**e 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 h**e made such a m**take.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．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）
3、表示对将来情况的主观推测
从句：①if+主语+were to do（if 表示未来的虚拟语气独有形式，其他如w**h，even if等词都没有该形式来表示未来的虚拟语气）
②if+主语+should+do
③if+主语+过去式（be动词用were）
主句：主语+should/would/might/could+do
eg：
1．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）
2．if there were a he**y 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）
3．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、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，而主句的动作与正在发生的事实不符。eg：
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 **rmed 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。如：
if he were free today,we would h**e 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 h**e greeted her.
要是他认识她的话，他肯定会去问候她了。5、当虚拟条件句的谓语动词含有were,should,had时，if可以省略，这时条件从句要用倒装语序，即把were,should,had等词置于句首，这种多用于书面语。eg：
should he agree to go there,we would send him there.
要是他答应去的话，我们就派他去了。were she here,she would agree with us.
如果她在这儿的话，她会同意我们的。had he learnt about computers,we would h**e hired him to work here.
如果他懂一些电脑知识的话，我们已经聘用他来这里工作了。6、非真实条件句中的条件从句有时不表达出来，只暗含在副词、介词短语、上下文或其他方式表示出来，这种句子叫做含蓄条件句，在多数情况下，条件会暗含在短语中，如without…，but for…（要不是因为.）等
eg：
but for h** help,we would be wo**ng now.
要不是他的帮助，我们还会在工作呢。without your instruction,i would not h**e made such great progress.
要是没有你的指导，我不会取得如此大的进步。we didn't know h** telephone number,otherw**e we would h**e telephoned him.
我们不知道他的电话号码，否则我们就会给他打电话。7、有时，虚拟条件语气的从句中，主、从句可以省略其中的一个，来表示说话人的一种强烈的感情。①省略从句
he would h**e fin**hed it.他本该完成了。you could h**e passed th** exam.你本能通过这次考试的。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.我要是得到它了该多好啊。8.注意，在虚拟语气的从句中，动词be的过去时态一律用were，不用was。eg：if i were you,i would go to look for him.
如果我是你，就会去找他。