1．表示想像或猜测
If I could speak French, I would teach you. 如果我会说法语，我就能教你。(我不会，所以不能教你)
If you phoned him right now, you might get the matter settled. 如果你现在给他打电话，你就可以把问题解决了。
There could be something wrong with the tape recorder. 这台录音机可能出毛病了。
He might have said so. 他可能这样说过。
Could he have done such a foolish thing 他会做这样的傻事吗？
2．后接不用连词的that从句
I'd rather you went home now. 我希望你现在就回家。
I would rather my daughter attended a public school. 我希望我的女儿能上公立学校。
C．表示愿望的感叹句
在表示祝愿的感叹句中，用动词原形，表示愿望。
Long live the People's Republic of China! 中华人民共和国万岁！
May you be happy! 祝您快乐！
God bless you! 上帝保佑你！
Success attend you! 祝你成功！
The Lord save us! 愿主救我们！
D．情态动词用于虚拟语气
部分情态动词的过去形式(could, might, should, would)，可以用于非真实条件句以及其他结构中表示虚拟语气。
A． If only......
If only位于句首引起的感叹句用虚拟语气，动词用一般过去时表示目前的愿望，用过去完成时表示过去的愿望，用would或could表示将来。
If only he knew the answer. 他要是知道答案就好了。(用过去时表示现在)
If only I could speak several foreign languages！我要是能讲几种外语就好了！(用would/could表示将来)
If only you had told me the truth before. 要是你以前告诉我真相就好了。(用过去完成时表示过去)
注意：if only和wish的用法相同，表示的意愿也基本相同。
If only it would rain. 但愿天能下点儿雨！= How I wish it would rain.
If only I had known her earlier! 要是我早点儿认识她就好了！= I wish I had known her earlier.
B．would rather
would rather, would prefer, would sooner 等后接从句，表示“宁愿”，动词用过去时表示当时和将来的情况，用过去完成时表示过去的情况。
I would rather you left today. 我宁可你今天走。
I would prefer he didn't stay here too long. 我倒希望他不要在这儿呆得太久。
I would rather I hadn't seen that film yesterday. 我宁愿昨天没有看那场电影。
提示：
would rather 主要有两种用法。
1．后接不带to的不定式
I'd rather play tennis than swim. 我宁愿打网球，也不愿游泳。
I'd rather not go to the movies. 我宁愿不去看电影。
Which would you rather have, tea or coffee 你喜欢喝茶，还是咖啡
3．表语从句和同位语从句中的虚拟语气
在表示建议、劝告、命令等含义的名词后的表语从句和同位语从句中，谓语动词要用“should + 动词原形”结构。
My advice is that you (should) practise speaking English as often as possible.我的建议是你尽可能经常地练习说英语。
The demand is that the composition (should) be written on one side only. 按照要求作文必须单面誊写。
It is my desire that all the members of the family (should) gather once a year. 我的愿望是所有家庭成员一年团圆一次。
The suggestion that education (should) be reformed is reasonable. 教育要改革的建议是合理的。
必背：
表示建议、劝告、命令等含义的名词常见的有
advice 建议
demand 要求
desire 愿望
decision 决定
idea 意见
motion 提议
order 命令
proposal 提议
requirement 要求
request 请求
regulation 规章
suggestion 建议
B．状语从句中的虚拟语气
①方式状语从句
由as if 或as though引导的状语从句可以用陈述语气表示可能符合事实的情况，也可以用虚拟语气表示与事实不符或与事实相反的情况。
a. 表示与现在事实不相符，用一般过去时
The teacher treats the pupil as if he were her own child. 这位老师对待这位学生就像自己的亲生孩子一样。(这位学生并不是她的亲生孩子)
b. 表示与过去事实不相符，用过去完成时
I felt as though we had known each other for years. 我感觉我们好像认识多年了。(其实并没有认识多年)
c. 表示与将来事实不相符，用would/might/could
It looks as if it might rain. 天看上去好像要下雨。(不大可能下雨)
对比：
He looks as if he is young. 看样子他好像很年轻。(他就是年轻)
He looks as if he were young. 看样子他好像很年轻。(实际上他不年轻)
②目的状语从句
a. 由in case, lest, for fear that引导的目的状语从句，动词用“should + 动词原形”结构，表示忧虑或目的。
Please remind me of it again tomorrow in case I (should) forget. 请你明天再提醒我这件事，以免我忘记。
She emphasized it again and again, lest he (should) forget. 她一再强调这一点免得他忘记。
We had a meeting and talked the matter over face to face for fear that there
should be any misunderstanding.
我们开了一个会，面对面谈了这件事，以免发生任何误会.
b. 在in order that和so that引导的目的状语从句中，往往用can(could)或may(might)。
I shall write down your telephone number so that I may not forget. 我要把你的电话号码记下来，以免忘记。
They worked harder than usual in order that they could finish the work ahead of time. 为了能提前完成工作，他们比往常更加努力。
We will tell you the truth so that you can judge for yourself. 我把真实情况告诉你，使你能自己作出判断。
③让步状语从句
让步状语从句指事实时，从句谓语动词用陈述语气。若从句内容表示现在和将来的假设情况，从句谓语动词用虚拟语气。
Though he (should) fail, there would still be hope. 即使他失败了，仍有一线希望。
I should say the same thing even if he were here. 即使他在这里，我还是要这么说。
Whether he (should) succeed or fail, we shall have to do our part. 不管他成功还是失败，我们还是要做好自己的事。
However hard it might rain rains, we shall go there together. 不管雨下得多么大，今晚我们还是要去那里。
No matter what his social position (might) be, a man is equal in the eye of the law. 一个人不论其社会地位如何，在法律面前都是平等的。
C．定语从句中的虚拟语气
在It is (high) time (that) ...结构中，定语从句的谓语动词用过去式，表示“到某人该做某事的时间了”。
It is time that the children went to bed. 到孩子们睡觉的时间了。
It is high time that we began the meeting. 正是我们开会的时间了。
2．宾语从句
虚拟语气常用在表示要求、建议、命令或愿望的动词后的宾语从句中。
常见的宾语从句中需用“should+动词原形”的动词有：
一个坚持：insist
两个命令：order, command
三个建议：suggest, advise, propose
四个要求：demand, require, ask, desire
①在suggest(建议), order(命令), propose(建议), insist(坚持要做), command (命令)，request(要求)，advise(建议)等动词后的宾语从句中，谓语动词用 should + 动词原形 (美国英语常省略should)。
The examination instructor asked that the students (should) not use a calculator. 考官要求学生不要使用计算器。
The workers demanded that their wages (should) be raised by 10 percent. 工人们要求增加百分之十的工资。
They requested that he (should) sing a song. 他们要求他唱一首歌。
比较：
动词suggest和insist后面的宾语从句，应根据不同情况选用陈述语气或虚拟语气。
suggest作“建议”解时，用虚拟语气，作“使想起、暗示”解时，用陈述语气。
His silence suggested that he agreed with my decision.他的沉默暗示他赞同我的决定。(暗示)
He suggested that I (should) stick to my decision.他建议我坚持自己的决定。(建议)
insist作“坚决要求”解时，用虚拟语气，作“坚持认为”解时，用陈述语气。
He insists that doing morning exercises does good to people's health. 他坚持认为做早操对健康有益。(强调)
He insists that he (should) do morning exercises every day.他坚持每天都要做早操。(要求)
②在动词wish后的宾语从句中用虚拟语气，表示不可能实现的愿望。
一般过去时were/did 表示目前不可能实现的愿望
I wish I were a bird. 但愿我是一只小鸟。
I wish I knew the answer.我要是知道答案就好了。
过去完成时 had done 表示过去不可能实现的愿望
I wish she had taken my advice.那时她要是接受我的建议就好了。
I wish that I had seen her yesterday.真希望我昨天见过她。
过去将来时 would/could do表示将来没有把握或不可能实现的愿望
I wish you would stay an hour longer. 我希望你再呆一个小时。
I wish it could stop raining.但愿雨能停。
由于wish和hope的汉语意思差不多，在使用上往往容易出错。wish一般表示“希望”、“但愿”，表示不可能实现的愿望，后接从句时要用虚拟语气；hope表示“希望”，指可以实现的希望，后接从句时用陈述语气。
The students hope that their football team will win the game. 学生们希望他们的足球队获胜。 (可能实现)
I wish I could see him now. 我希望现在就能看到他。(不可能实现)
虚拟语气除主要用于非真实条件句外，还可用在部分主语从句、宾语从句、表语从句、同位语从句、定语从句和状语从句之中。
A．名词性从句中的虚拟语气
主句中的动词、名词、形容词如表示要求、建议、命令或愿望等含义，与其相关的名词性从句(主语从句、宾语从句、表语从句和同位语从句)中的谓语动词要用虚拟语气。
1．主语从句中的虚拟语气
主语从句通常用于“It is + 形容词名词动词的-ed形式 + that ...”结构。主语从句中是否用虚拟语气取决于所用的形容词或动词的-ed形式，其谓语动词常用 “should + 动词原形”(美国英语常省略should，只用动词原形)。
①表示建议或命令
It is important that we (should) work out a study plan. 重要的是我们要制订出一个学习计划。
It will be better that we (should) meet some other time. 最好我们在另一个时间见面。
It is suggested that each student (should) sing a song in English. 有人建议每个学生都要唱一首英文歌曲。
It is demanded that he should leave at once. 要求他立刻离开。
②表示惊讶
It is strange that he should have failed to see his own shortcomings. 真奇怪他竟看不到自己的缺点。
It is a pity/shame that I should be at school instead of lying here in hospital. 真可惜，我应当在学校而不是躺在医院里。
It is natural that you should forget it first. 你起先记不住是很自然的。
常见的要接含有虚拟语气主语从句的词
形容词：important, necessary, urgent, essential, vital, natural, strange, proper等
动词的-ed形式：arranged, suggested, ordered, required, desired, demanded, proposed等
It is necessary that the program (should) be loaded into the computer. 有必要把程序输入电脑。
It is strange that he should have learned so much in such a short time. 很奇怪他竟然在如此短的时间内学了那么多的东西。
It is desired that we (should) get everything ready by tonight. 希望一切在今晚前准备就绪。
3．条件用其他形式来表示
She was ill, otherwise she would have been present at the meeting. 她病了，否则的话就会出席会议了。(条件通过连词otherwise/or表示出来 If she had not been ill...)
I told him to go there himself, but perhaps I should have gone together with him. 我让他自己去那里，但也许我应该和他一道去。(连词but暗示条件)
Suppose you were in my shoes, what would you do 假若你站在我的立场上，你会怎么办？(用suppose/supposing/providing等词表示if)
To talk with her, you would know she could not hear well. 如果你同她交谈，你就会知道她的听力不好。(动词不定式带有假设的意思，相当于 If you should talk with her...)
Left to himself, he could not have finished the work. 要是放任他的话，他是不可能完成工作的。(动词的-ed形式表示条件 If he had been left to himself...)
在很多情况下，虚拟式已变成习惯说法，很难找出其暗含的条件。
You wouldn't know. 你不会知道。
I would like to go with you. 我愿意和你一起去。
2．条件可根据上下文推理出
It would do you no good. 这可能会对你没好处。(条件可能是if you should give up the job 如果你放弃这项工作的话)
They could have won. 他们本来是会赢的。(条件可能是if they had been patient 如果他们有耐心的话)
Such mistakes could have been avoided. 这种错误完全能避免。(条件可能是if you had been more careful 如果你更加小心一点的话)
Why didn't you tell me about it I should have helped you. 为什么你不告诉我？我会帮助你的。(条件可能是if you had told me about it 如果你当时告诉我的话)
--- Did you go to see him yesterday 你昨天去看他了吗？
--- I would have, but someone dropped over to my house for a visit. 原想去的，但有人到我家来玩了。(条件是if no one had dropped over to my house for a visit 如果没人到我家来玩的话)
1．条件暗含在短语中
He would not get such a result without your help. 没有你的帮助，他就不会有今天的成果。(条件暗含在介词短语without your help中)
But for you, I could not be recovered so soon. 要不是有你，我就不会恢复得这么快。(条件暗含在But for you中)
This same thing, happening in the past, would lead to a disaster. 同样的事，如发生在过去，就会酿成大祸。(条件暗含在分词短语happening in the past中)
He must have been there, or he never could know the place so well. 他一定是去过那儿，否则他绝不会对那个地方如此熟悉。(暗含条件是连词or)
3． 表示在将来不太可能实现的条件
表示在将来不太可能实现的条件从句有三种形式：
① weredid
条件从句用动词的过去式(be动词用were)，主句用should(第一人称)或would(全部人称)+动词原形。也可用could, might代替should, would。
If he were here tomorrow, I would speak to him. 明天如果他在这里的话，我就和他谈谈。
If you dropped the glass, it would break. 你要是把杯子掉下来，会打碎的。
If she had time, she could help me. 她要是有时间，就会帮我了。
② should do
条件从句中不管什么人称都用should do，可表示有偶然实现的可能性。
If it should rain, the crops could be saved. 假如下雨，庄稼可能就有救了。
We would trust him if he should be honest. 如果他真是诚实的，那我们就相信他。
③ were to do
条件从句用were + to do。这种形式比较正式，常出现在书面语中，其假设成份很大，实现的可能性很小。
If I were to do the work, I should do it in a different way. 要是我做这项工作，我就会以不同的方式去做。
If the headmaster were to come, what would we say to him 假如校长来了，我们对他说什么呢？
将来不太可能实现的三种非真实条件句中的虚拟语气对比
If it snowed tomorrow, I would stay at home. 如果明天下雪的话，我就留在家里。
(常用形式)
If it should snow tomorrow, I would stay at home. 如果明天下雪的话，我就留在家里。
(可能性较小)
If it were to snow tomorrow, I would stay at home. 如果明天要下雪的话，我就留在家里。(可能性最小)
C．省略if的条件句
在书面语中，非真实条件句中有were, had, should时，可以省略if，而把were, had, should放在主语前，用倒装结构。
Were it necessary, I might go without delay. 如果需要的话，我可以立即去。(= If it were necessary......)
Had you taken my advice, you wouldn't have failed in the exam. 你要是听了我的建议，就不会考试不及格了。(= If you had taken my advice......)
Should I have time, I would call on her. 要是有时间，我就去看她。(= If I should have time......)
D．错综条件句
虚拟条件句中的主句和从句涉及的动作发生的时间不一致，这时主句和从句的谓语形式应按照各自动作发生的实际时间来表达。
If you hadn't watched TV yesterday, you wouldn't be so sleepy now. 如果你昨天不看电视，你现在也就不会这么困了。(从句yesterday说明过去，主句now说明现在)
If they had left home early this morning, they would arrive in half an hour. 如果他们今天一早就离开家的话，再过半小时他们就该到了。(从句说明过去，主句说明将来。)
If you hesitated this moment, you might suffer in the future. 如果你此刻犹豫不决，你将来会吃苦头的。(从句说明现在，主句说明将来)
注意：
在很多情况下，我们也可以用was来代替当主语是第一人称或第三人称单数时的were。
If were here tomorrow...
也可以说成：If he was here tomorrow...
I wish I were a bird.
也可以说成：I sish I was a bird.
但倒装句型中的were不可被was来替换。
Were I in your position,I would not do it.如果我处在你的位置，我是不会干这件事的。
E．含蓄条件句
非真实条件句中的条件从句有时不表达出来，只暗含在上下文中，这种句子叫做含蓄条件句。含蓄条件句大体有三种情况：