他会知道怎样把机器修好的。(表示愿望)
He suggests that we should all go to see the film.他建议我们都去看电影。(表示建议)
虚拟语气的动词形式 比较特殊，共有下列七种： 1)动词原形(用于一切人称和数)
2)动词的过去式(用于一切人称和数，be的过去式用were) 3) had + 过去分词(用于一切人称和数) 4) should + 动词原形(用于一切人称和数) 5) should have + 过去分词(用于一切人称和数)
6) should (第一人称)，Would(第二、三人称) + 动词原形 7) should(第一人称)，Would(第二、三人称) + have + 过去分词
[注]上述某些动词形式和直陈语气的某些动词形式相同，但它们的用法及其所表示的时间概念则完全两样，二者切不可混淆。
虚拟语气常用在表示条件的从句和表示结果的主句中。 虚拟语气在条件从句和结果主句中的用法 1)表示与现在事实相反的条件和结果，：如：
If I Were you， I should go and see the dentist at once. 假如我是你的话，我会马上去看牙科医生。
If they were no gravity, we should not be able to walk. 假如没有引力，我们就不能行走。
If they had time，they would study Italian too. 假如他们有时间的话，他们也会学
意大利语的。
If She knew German，She would read Marx and Engels in the original假如她懂德语，她就会读马克思和恩格斯的原著了。
He would tell me if he knew， but he does not，know anything about it. 假如他知道的话，他会告诉我的，但是他一点也不知道。
2)表示与过去事实相反的条件和结果。如：
If I had known of your arrival I should have met you at the station.如果我早知道你要来，我会去车站接你的。
If you had come a few minutes earlier，you would have met him.要是你早来几分钟的话，你就见到他了。
If She hadn't been so strict with herself, she wouldn't have made such great progress.她要是对自己要求不严格，她就不会有这样大的进步。
If the child had fallen through the ice, he would have drowned. 假如这孩子掉到冰窑里，他定会淹死了。
3)表示与将来事实可能相反的条件和结果。其用法和表示与现在事实相反的条件和结果相同。如：
If it Were Sunday tomorrow，my brother would go skating at the Beihai Park.如果明天是星期天，我弟弟会去北海公园滑冰的。
If you dropped the glass，it would break.你如将杯子掉下来，它就会打碎的。 What Would happen if I put the paper on the fire? －It Would bum.我若把纸放在火上会怎么样?--纸就会烧着。
[注]在表示与将来事实可能相反的条件从句中，亦可用were to + 动词原形(比较正式，
常用于书面体中)和should + 动词原形。如：
If you were to come tomorrow，I might have time to see you.你如果明天来的话，我或许会有时间见你。
If I were to see her tomorrow，I would tell her about your decisions. 我明天如见到她，就把你的决定告诉她。
If it should rain tomorrow，what would we do?明天万一下雨，我们怎么办? If you should meet George，tell him I want to see him.你如见到乔治，告诉他我要见他。
[英语语法手册]虚拟语气 B 不用if的条件从句结构
在笔语中，条件从句有时可以不用连词小而把were，had或should移至主语之前。但如从句没有were，had或should，则一般不能这样做。如：
Should he come，tell him to ring me up.他要是来了，让他给我打个电话。 Were I in your position，I would go.如果我处于你的地位，我就去。
Had they not helped us，our experiment would have failed. 如果没有他们的帮助，我们的试验是会失败的。
[注]有时虚拟条件不用条件从句而用介词短语、动词不定式等来表示。如： without contradiction nothing would exist. 没有矛盾就没有世界。
We could not have done the work well without your help.要是没有你的帮助，我们不可能做好这工作。
It wonld be a good idea to go swimming at the summer palace.到颐和园去游泳，
倒是个好主意。
All this would have been impossible ten years ago.这一切在十年前是不可能有的。
[英语语法手册]虚拟语气 C 虚拟语气和情态动词
虚拟结构(不论从句或主句)常可用情态动词的过去式，即could，might，wou1d等加不带to的动词不定式或不带to的动词不定式完成式，来作谓语动词。这些情态动词除表示虚拟结构外，本身还有独立的意思。如：
If I were you，I Would go.我如果是你，我就愿意去。
If he were here，he might agree with you.假如他在这里，他可能会同意你。 I Would help you if I could. 假如我能够的话，我一定帮助你。
If it had not been for their help，we could not have succeeded. 如果没有他们的帮助，我们是不可能成功的。
[英语语法手册]虚拟语气 D
有时条件从句中的动作和结果主句中的动作所发生的时间不一致 这时，动词的形式应根据它所表示的时间加以调整。如：
If he had followed the doctor's advice，he Would be quite all right now.他当时若是听医生的话，现在就会痊愈了。(从句说明过去，主句说明现在)
If I were you，I Would have gone to the theatre.假如我是你，我会去看话剧了。(从句的时间包括现在，主句说明过去)
If you hadn't helped me，I couldn't have finished it on time and most likely I
would be still working now.假若你没有帮助我，我不可能按时完成工作，很可能现在还在干哩。(从句说
明过去，主句说明过去和现在) 用直陈语气表示条件和结果
在现代英语里，多用直陈语气表示条件和结果。直陈语气的条件和结果所表示的往往是事实。如不是事实，其可能性也较虚拟语气要大。如：
If it rains tomorrow，we shall not go out.假如明天下雨的话，我们就不出去。 The dog won't attack you if you sit still.如果你安静地坐着，狗是不会咬你的。 If the train leaves at eight thirty，there is no time to lose. 火车如果是八点半开，我们得抓紧时间了。
If the rain stops，I'll go for a swim.要是雨停了，我就去游泳。
[英语语法手册]虚拟语气 E 省去从句或主句的虚拟结构
虚拟结构中的从句或主句有时形式上可以省略，但意义上却仍存在。 1)省去了条件从句的虚拟结构。如：
That would be fine.那就太好了。(省去了if you should do it或类似条件)
You might stay here forever.你可以永远呆在这里。(省去了if you wanted协或类似条件)
I would not have done it.我是不会做那件事的。(省去了if were you或类似条件) [注]虚拟结构中省去的从句有时并不容易找出。如： Who would have thought of it ? 谁会想到是这样的呢?