我们知道由if 引导的条件句有两种形式，一种是真实的条件句，也就说是某事很可能发生，条件是可能存在的，主句中某种情况发生的概率也是很高的。例如If you intend using humor in your talk to make people smile, youmust know how to identify shared experiences and problems。(2002。Text1 )(如果你想在谈话中用幽默来使人发笑，你就必须知道如何识别共同的经历和共同的问题。)
If + did / were + ……, …… would
另一种就非真实条件句，通过虚拟语气来表达。例如Iwould have arrived much earlier if I had not been caught in the traffic。(要不是交通堵塞，我本会来得早一些。)在这句话中，假设“不堵塞”，但是真实情况并不是这样，所以用虚拟语气。一般有6种常见的形式：
1)与现在事实相反的非真实条件句，从句中用一般过去时，主句中用would/could/ should 加动词原形，be 动词的过去式为were。
could + do (动词原形)
If you then examined theEuropean national youth teams that feed the World Cup and professional ranks,you would find this strange phenomenon to be ever more pronounced。(2007,text 1)(查看为世界杯和专业队输送人才的欧洲国家青年队，你会发现这一奇怪的现象更加明显。)
2)与过去事实相反的非真实条件句，从句用过去完成时，主句用would/could/should +have done 。
If I were you, I would go abroad at once. (I am not you.)
If he had played lastseason,however,he would have been one of 42. (2008 text3 )(如果他打了上个赛季，他将只是42个(最高的人)中的一个)
If he knew it now, he could help me. (He doesn't know it now.)
3)与将来事实相反的非真实条件句，从句用should do 或 were to do 结构，主句中用 would /could/should 加动词原形。
If you were to examine thebirth certificates of every soccer player in 2006’s World Cup tournament, youwould most likely find a noteworthy quirk: elite soccer players are more likelyto have been born in the earlier months of the year than in the latermonths。(2007 text 1)(如果查看2006年世界杯联赛每个足球运动员的出生证，你很可能发笑一个值得注意的奇怪现象：好的足球运动员都在一年的头几个月而不是后几个月出生。)
If + had done + ……, …… would (might) have done ……
If I had known your telephone number yesterday, I would have phoned you.
(I didn't know your telephone number.)
If he had put all theimportant files in the safe yesterday, his boss wouldn’t be scolding him soangrily now。(如果昨天他把所有重要文件都放在保险箱中，他的老板现在也不会这个严厉的说他。)
If you had come here a littleearlier just now, you might have met her.
(You didn't come here earlier.)
Had it not for the timelyinvestment from the general public, our company would not be so thriving as itis。(1998 )(如果不是群众投资，我们的公司不会像现在这么兴旺。)
6)用介词或连词表示虚拟。比如with,without, under…condition, but for 。
(1) If + should + v, …… would + v.
And if one received agrape without having to provide her token in exchange at all, the other eithertossed her own token at the researcher or out of the chamber, or refused toaccept the slice of cucumber。(2005 text 1 )(如果一只猴子甚至没有用石块就能得到葡萄，另一只猴子就会向研究人员扔石头，或者把石头扔出房间，或者拒绝接受一片黄瓜。)
If it should rain tomorrow, you could stay at home.
If I should fail, what should Ido?
(2) If + did / were to + v ……, would + v.
If the sun were to rise in the west, I would lend you the money.
If you finished it in 3 minutes, I would give you my car.
● 下列动词后的“宾语从句”中需要用虚拟形式，即should + 动词原形，shoud在美国英语中要省略(TOEFL语法考点)。此类常见的动词有