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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。英语作文
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