1、对现在情况的虚拟
条件从句中谓语动词用一般过去式，主句用“would/should/might/could+动词原形”形式，表示对现在不可能实现情况的假想。
If I were you,I would choose to work in a small town.
如果我是你，我会选择在小城镇工作。（事实上我不是你）
注意：在条件从句中，如果谓语动词是be的形式，不管主语是单数还是复数，书面语中用were,而口语中如果是单数，也可以用was,不过在if I were...中一般用were.
2、对过去情况的虚拟
条件从句中谓语动词用过去完成式，主语用“would/could/should/might+have+动词的过去分词”形式。
If it had not been for your help yesterday,I could not have caught the bus.
如果昨天没有你的帮助，我不会赶上公交车。（事实是你昨天已经帮助了我，所以我赶上了车)
3、对将来情况的虚拟
在表示与将来事实相反的条件从句中，谓语动词可用should do,were to do或动词的过去式三种形式表示，但意义略有不同。should表示的可能性最大，动词过去式的可能性次之，were to do 的可能性最小。
If things were to be twice，all would be wise.
假如凡事都可以重新做一次，人人皆可成为聪明人。（事实是并不是所有的事都可以重新做一次)
注意：在书面语中，如果虚拟条件从句中有were，had 或 should，可以把if省略，把这几个词放到主语之前，构成主谓倒装。
扩展资料
其他条件句：
1、错综时间条件句
有些条件句的主句谓语和从句谓语表示的动作在时间上并不一致，这类句子称为错综时间条件句。
If you had taken my advice then,you would not be in trouble now.
如果当时听我的劝，你现在就不会有麻烦了。(if 条件句是对过去的虚拟，故用过去完成时，而主句是对现在的虚拟，故用“would+动词原形”形式）
2、含蓄条件句
有些句子虽不含条件从句，但意思和条件句相差不多，这种句子称为“含蓄条件句”，这种句子有时候会用虚拟语气。
如: Without air (If there were notair), there would be no living things.
But for your help (If it ha dn'tbeen for your help) I couldn't have don e it.
1、表示与现在事实相反的情况
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）
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+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）
3、表示与将来事实相反
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: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）
本回答被网友采纳例如：
If he doesn't hurry up,he will miss the bus.如果他不快点，他将错过巴士。( 真实)
If he is 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 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）
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+have done
eg：
1． 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）
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．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wish， 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 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）
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 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。
如：
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。
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 have 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 his help,we would be working now.
要不是他的帮助，我们还会在工作呢。
Without your instruction,I would not have 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 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。
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 have finished it. 他本该完成了。
You could have passed this 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。本回答被网友采纳
表示将来可能发生的事情，也就是假如将来怎么样，主语就会做什么事情。
例1：If it rains, I'll stay home.
时态：主将从现
例2：If it rains, I might stay home.
时态：主情态动词+动词原型，从现
表示当下可能发生的情况，也就是假如以前怎么样，现在就会怎么怎么样。
例：If I studied more, I would get good grades.
这里说明我现在已经考砸了，表达的意思是如果我之前再更努力一点，现在的分数会 考得更好。
I wouldn't tell her if I were you. 如果我是你，我就不会告诉她。虽然从句用的是were，但是讲的是现在可能发生的事情。需注意，if从句中的be不管主语是谁，统统用were!!!
时态是主情态动词+动词原形，从过。这里情态动词必须，且只能是could, shoud, would, might，不能是can, shall, will, may。
表示过去发生的情况，如果过去怎么怎么样，那么以前就会怎么怎么样。
例：If I had been feeling well, I would have been in class. 如果我身体撑得住，我之前就会来校了。
时态是主情态动词+现在完成时，从过去完成时。
如：She may come if you invite her.如果你邀请她会来。
If you ask him,he will hlep you.如果你请他帮忙，他会帮你的。
If you fail in the exam,you will let him dowm.如果你考试不及格，你会让他失望的。
