4. if条件句中的虚拟语气形式
1）if 非真实条件句所表示的假设则是不可能或不大可能发生或实现的，句中的主句与从句都用虚拟语气。
在if非真实虚拟条件句中，主句和从句谓语动词主要有下面几种形式：

假设类型 条件从句谓语动词形式 主句谓语动词形式
与现在事实相反 动词过去时（be 用 were） Should（would，could，might） + 动词原形
与过去事实相反 Had + 过去分词 Should（would，could, might）+ have + 过去分词
与将来事实可能相反 动词过去时（should+动词原形，were to + 动词原形） Should（would，could，might）+ 动词原形

If there were no gravity, we should not be able to walk.
假如没有引力，我们就不可能行走。
I wouldn’t have known what these were for if I hadn’t been told.
假如别人不告诉我，我就不知道这些东西是干什么的了。
2） 正式文体中，有时可把虚拟条件句中的连词if省去，而将were，had, should等助动词（不包括行为动词）提到主语前面。如果句中没有were，had或should时，既不能省略if，也不能倒装。例如：
Were I to meet him tomorrow (= If I were to met him tomorrow), I should ask him about it.
要是我明天见到他，我就会问他这件事的。
Had I had the money last year (= If I had had the money last year), I would have bought the house.
如果我去年有了这笔钱，我就买那所房子了。
Should there be any trouble with the boiler, the automatic controlling unit would cut off the fuel oil supply.
假如锅炉出问题的话，自控装置会自动切断燃油的供给。
3） 通常情况下，在非真实条件句中主句和从句的谓语动词所指时间是一致的，但有时也可能指不同的时间，这时要根据上下文的意思采用不同的谓语动词形式。例如：
If I were you, I would have taken his advice.
我要是你，我就采纳了他的建议。（从句指现在，主句指过去）
If the weather had been more favorable, the crop would be growing still better.
如果气候更适宜一些，庄稼会长得更好。（从句指过去，主句指现在）

5. 含蓄虚拟条件句
1）有时假设的情况并不以条件从句形式表示出来，而是通过一个介词短语或其他方式表示。常用的词或短语有：without, but for, but that, otherwise, or, but 等。例如：
Without your help (=If we had not had your help), we could not have succeeded.
要是没有你的帮助，我们就不会成功的。
But for electricity (= If there were no electricity), there would be no modern industry.
要是没有电，就不会有现代工业。
He was having a meeting; otherwise he would have come over to help us.
他当时正与他的学生进行讨论，否则的话他就来帮我们了。
He felt very tired yesterday, or he would have attended the party.
他昨天很累，不然他就参加那个聚会了。
2）在某些暗含虚拟条件的简单句中，虚拟语气可通过上下文表现出来。谓语动词用should/ would be 形式和should/ would have been 虚拟形式。例如：
Any men in his position would have done like that.
任何人处在他的位置都会那么做的。
You should (ought to) have come earlier.
你本应来得早点。

6. 其他句型中的虚拟语气
1）在would rather, would sooner, would just as soon, might as well和would prefer等后面所跟的从句中，也可以用虚拟语气，表示愿望，意为“宁愿、但愿”。其形式为：
①“would rather (would sooner…) + 主语 + 谓语动词过去式” 表示现在或将来的情况。
②“would rather (would sooner) + 主语 + 动词过去完成式”表示过去的情况。例如：
I’d rather he didn’t go now.
要是他现在不走就好了
I’d just as soon you had been here yesterday.
要是你昨天在这里就好了。
2）在句型“It is (high/ about) time…”后面也可以跟虚拟语气，从句中常用过去式。有时也用“（should）+ 动词原形”，意为“该是……的时候了”。例如：
It is time that the boy (should) go to school.
这个小孩该上学了。
It is high time (that) the weather improved.
天气真该好起来了。
3）在“if only”引起的感叹句中也要求用虚拟语气，表示愿望。
形式为：①用过去时或“would/ could + 动词原形”表示与现在或未来事实相反的愿望。
②过去完成时表示与过去事实相反的愿望。例如：
If only you would listen to our advice.
要是你听我们的建议就好了。
If only I had not been busy last week!
要是上周我不忙该多好啊！
If only she could marry me.
但愿她能嫁给我。
注：if only也可以用于虚拟条件句中。例如：
If only I had more money, I could buy a car.
要是我有更多的钱，我就可以买辆车了。
4）连接词“in case, lest, for fear that”可以用来引导虚拟语气。其形式通常为：“…lest (in case, for fear that) + 主语 + should + 动词原形”。Lest, for fear that 句中的 should 可以省去， in case 句中的 should 通常不省去，但是 in case 句中可以不用虚拟语气，而用陈述语气。Lest, for fear that后面也可以接其他形式。例如：
He took his coat with him in case it should rain.
他带着雨衣以防下雨。
I will not make a noise for fear that I (should/ might) disturb you.
我不会做声的，以免打扰你。
Care must be taken in using this method lest overflow (should) occur.
在使用这以方法的时候要谨慎小心，以免发生溢流现象。