1.在表示与现在的情况相反的虚拟条件句里,if从句用过去式,主句用“would/should/could/might+动词原形”。
If I had a mini-computer on hand， I would certainly let you use it. 如果我现在手头有一台微电脑,我肯定会借给你使用。
If I knew Japanese， I could be able to interpret what our director says into Japanese.如果我懂日语我就能 把厂长的话翻译成日语了。
在虚拟的if从句里,如果动词是be,通常用 were; 在第一，三人称单数主语后可以用 was 或 were，但 were 更为正式，特别是在陈述心存怀疑的事情时:
If places alike ，there would be little need of geographers.
A） being B） are C） be D） were
如果各地方都一样,就不需要地理学家了。(1991.6.-4， 答案是D。)
If I was /were better qualified， I'd apply for the job. 如果我的条件好一些,我就申请那份工作了。
在陈述想象的事情时,were 更为常见:
If I were a bird， I'd fly to you at once. 如果我是一只鸟,我就立即飞到你的身边。
但总是说“if I were you.../ if I were in your position ”,这些说法经常用来提出建议:
If I were you / If I were in your position， I would try my best to learn a second foreign language. 如果我是你,我会尽力学习第二门外语。
“if it were not for...” 意思是“如果不是……“，用来描述某事已经发生或没有发生:
If it were not for your help， I would still be homeless. 如果没有你的帮助,我现在仍然是无家可归。
2.在表示将来难以实现的假设或推测的虚拟条件句中,动词用过去式,或“were+to 不定式”,或“should+动词原形”;主句用“would/should / could / might + 动词原形”。例:
Jean doesn't want to work right away because she thinks that if she a job， she probably wouldn't be able to see her friends very often.
A） has to get B） were to get
C） had got D） could have got
吉恩不想立刻工作,因为她想:如果她得到一份工作,她就不能经常去看她的朋友了。（B4，1996.1-44， B对)
If it should rain tomorrow， we should have to put off the picnic. 万一明天下雨,我们只好推迟野餐。
3.表示与过去的事实相反,从句用过去完成式,主句用“would/should/could/might+have+过去分词”。例:
I would have come to see you had it been possible， but I so busy then.
A） had been B） were C） was D） would be
（1999年全国大学生英语竞赛初赛题） C对。假如可能的话我早就去看你了,但我那时太忙了。
If the policeman had arrived earlier， he would have seen the acci-dent. 要是警察早些到达,就能见到事故了。
If better material had been used， the efficiency of the machine would have been much increased. 如果使用更好的材料,机器的效率会大大提高。
4.注意如果以上条件句中含有助动词、情态动词,be或have， 则常常不用if,而用倒装结构:
My cat wouldn't have bitten the toy fish it was made of rubber.
A） if she has known B） if she knew
C） she should have know D） had she known
如果我那只猫知道那条玩具鱼是用橡胶制作的,她就不会去咬它了。（B4，1988.6， 答案是D)
for my illness I would have lent him a helping hand.
A） Not being B） Had it not been
C） Without being D） Not having been
如果不是我病了，我会帮助他的。(B4，1995.1-47， B对）
before we depart the day after tomorrow， we should have a wonderful dinner party.
A） Had they arrived B） Would they arrive
C） Were they arriving D） Were they to arrive
如果我们后天走之前他们到达的话,我们就举行一次丰盛的宴会。（B4, 1997.1-34， D对）
5.有时候条件并没有用if从句表示出来:
You are really incompetent! It hours ago.
A） finished B） should be finished
C） must be finished D） should have been finished
（1999年全国大学生英语竞赛初赛题） D对。你真是无能!这工作你几小时前就该完成了。)
We didn't know his telephone number， otherwise we him.
A）would have telephoned B）must have telephoned
C） would telephone D）had telephoned
我们不知道他的电话号码;要是知道的话,我们早就打电话给他了。（B4，1995.6-46， A对)
He must have had an accident， or he then.
A） would have been here B） had to be here
C） should be here D） would be here
他一定出了事 ,否则他已经在这里了。(B4，1990.1-51，A对。该句相当于:If he hadn't had an accident， he would have been here then.)
6.有时候,条件句表示的动作,与主句表示的动作发生的时间不一致(如一个发生在过去,一个发生在现在),这时,动词的形式要根据表示的时间来调整，这种句子语法家称之为错综时间条件句:
If I hadn't stood under the ladder to catch you when you fell ， you now.
A） wouldn't be smiling B） couldn't have smiled
C） won't smile D） didn't smile
你掉下来的时候,如果我没有站在梯子下面接着你,你现在就不会笑了。（B4，1999.1-60， A对。）
Look at the situation I am in！ If only I your advice.
A） follow B） had followed
C） would follow D） have followed
看我现在的处境多么糟糕!要是我早听你的劝告多好。（B4，1993.6－70，B 对）
