爱华网一，if条件句中
if 引导的条件句中，如果所说的与事实相反，则要用虚拟语气。其具体的规则如下：
主句从句
与现在事实相反would/should/could+动词原形一般过去时
与过去事实相反would/should/could have done过去完成时
与将来事实相反would/should/could+动词原形 Should+动词原形/一般过去时/were to+动词原形
注意：虚拟语气中的be动词，一般用were，少用was
(1)与现在事实相反，从句谓语动词用did(be用were，少用was )，主句谓语动词用would(should,could,might)+do
If the weather were fine , they would go for a swim. ( In fact itis rainy ,so they won’t go for a swim, )如果天气好， 他们就去游泳了。
If we had time now , we should read it again . (In fact now we haveno time to read it again . ) 要是现在有空， 我们就把它再看一遍。
If John had his way [为所欲为], he would spend all of his time playingtennis. = John wants to play tennisoften如果让约翰由着性子来，他会把所有的时间都用来打网球的
If you dropped the glass , it would break . ( In fact the glasswouldn’t drop . )
(2)与过去事实相反，从句谓语动词用had done，主句谓语动词用 would(should,could,might)+ havedone
If he hadn't lived among the workers for so many years, he wouldn'thave been able to write such a goodnovel假如他没有和工人一起生活这么多年，他不可能写出这么好的小说
Mary could have gotten better grades if she had studied more= Marydid not get better grades because she did notstudy如果玛丽再刻苦一点，她会取得更好的成绩
The project would have been approved [批准；赞成]if it hadn't been forthe budget=The project was not approved because the budget washigh[高的 [预算].如果不是预算的原因，这个项目是会被批准
If we had needed to get in touch with them, we could have sent atelegram= We did not send a telegram because we did not need to getin touch with them. 如果我们当时需要和他们联系，我们会发电报的
If we had arrived on time, we would have gotten good seats= We didnot get good seats because we were late如果他主动向她提出邀请，她是会和他一起去的
We would have been here sooner if we hadn't gotten lost= We werenot here sooner because we got lost如果我们没有迷路的话，我们会早一点到达有
I should have died of hunger if the Communist Party had not come in1949=I am alive because the Communist Party came in 1949.要是一九四九年共产党不来的话，我是会饿死的
(3)与将来事实相反，从句谓语动词用did(should+do或were + to do)，主句谓语动词用would(should,could,might)+do。
If she should come , I would / should ask her for help .
If I were to go home tomorrow , I could have some deliciousdumplings . ( In fact I don’t have time to go home to eat dumplingstomorrow .Maybe your mother telephoned me that she would cookdumplings tomorrow )
注意： 在if 条件句中，还有其它用法
if引导的状语从句可以省略 if，而把从句中的动词 were, had 或 should移到主语前面，从句可摆放在主句前或后。
Were she younger, she would do it= She would do it were she younger. 如果她年青点， 她就会干的
Had he known her address, he would had gone to visit her.= He wouldhad gone to visit her had he known her address .如果他知道她的地址，他会去看她的
Should it happen , what would you do ? 如果 真发生这样的事， 你怎么办呢？
Were I you , I would go .
Should it be fine , we would go for an outing .
Had they time , they would certainly come and help us .
在虚拟语气的主句中，情态动词可用could(能够) , might(或许，可能) , should 替换would .意义有点不同。
If he were here , he might agree with you . 如果他在这儿，他可能同意你。
If it should rain , the crops could be saved . 如果下雨，庄稼很有可能得救。
If she should come , I would ask her for help .万一她来的话，我就请她帮忙。
3. 有时表示虚拟语气的条件从句或者主句都可以省略，而只剩下一个主句或者一个条件从句。但意义上却仍存在。(通过上下文及内在含义，句中往往有but, otherwise,unfortunately等类似转折词.这是含蓄虚拟。)
The old man’s hands shookconstantly . Otherwise he would be still working .这位老人的手不停地颤抖，否则他现在仍然在工作。
Our monitor was absent , or I would not have taken the chair.我们的班长缺席，否则我是不会主持会议。
He would have given you more help but he was verybusy.他会给你更多的帮助的但是他很忙。
I could help you. But I am so busy with doing my homework .我本来可以帮助你
She would have come to see you , but she was so busy that day.
I would have come earlier , but I didn’t know you were waiting.
I thought the children would be sleeping when we returned home ,but they were still awake .
If I had time. 我要有时间该多好啊
She should have come to the party. But unexpectedly a guest visitedher. 她应该来参加聚会。
If he had much more money. 如果他有更多的钱就能...。
4.虚拟语气中，条件从句的动词动作可以和主句的动词动作时态不一致,即混合虚拟。
If they had studied hard, they could do it easilynow.如果他们以前努力学习的话，现在就会容易些。 If he had not taken my advice, hewouldn't do it much better like this.
如果他不听取我的建议，他就不会干得这样好
If you were to master English , you should have worked hard .
If he had followed the doctor’s advice , he would be quite allright now .
I would have gone to visit him in the hospital had it been at allpossible, but I was fully occupied the whole of last week.(该句前半部分是假设虚拟，而后半部分是事实的陈述。)
If I had taken your advice , I should have no trouble with the worknow .
It it had rained last night , it would be very cold today .
5. 有时虚拟条件不用条件句if 而有介词短语表示。
Without air , there wouldn’t be living things in the world .
Without electricity , nothing could be done now .
But for you , I would be in hospital now . 要不是你， 我现在还在医院呢。
Without water , there would be no life. 没有水就没有生物。
With your help , we might finish the workearlier.有你的帮助，我们可能早点做完这工作。
But for your help , I would have failed.要不是你的帮助，我就会失败。
6．省去了结果主句的虚拟结果(常用以表示愿望)，一般在if only 句型中”要是…就好了”,和I wish虚拟表示形式一样.
If only I could help you .
If only the wind would stop .
If only I had attended yesterday’s lecture.
If only I had more money , I could buy a car .
7．If it be not for…”要不是….”也是一种条件虚拟语气从句。
if it were not for…与现在事实相反， if it had not been for与过去事实相反，两个都相当于butfor
If it were not for the guidance of the Party committee, we shouldfail. 如果不党委的指导，…
If it had not been for the help of our teacher , we should not havemade so much progress.
If it were not for the collective strength, it wouldn’t have beenpossible for us to live so well .
