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基本形式：If + had done…，…would /could / should /might + have done
例如：If she had been warned earlier，she wouldn't have broken the rules.
(事实上：She was not warned earlier and she broke the rules.)
If it hadn't rained，the match would have seemed more fascinating.
(事实上：It rained and the match was less fascinating.)
此种虚拟可进行倒装，如：
Had the letter been sent out，it would have ruined our friendship.= If the letter had been sent out，it would have ruined our friendship.
对将来事实的虚拟基本形式：
If + should do…，…would /could /should /might + do;意思类似汉语中的“万一”
例如：If he should forget the date，I might teach him a good lesson.
(事实上：他不大可能忘记那个日期)
If it should snow this afternoon，we could make a snowman.
(事实上不大可能会下雪)
此种虚拟可倒装，如：Should my car fail，I would have to return home on foot.= If my car should fail，I would have to return home on foot.
注意1：部分动词的宾语从句中需用虚拟语气，形式为should do，其中should常被省略。
此类动词有：insist，demand，suggest，propose，order，require，decide，ask，request等。
例句：We all insist that we (should) not rest until we finish the work.
The professor suggests that the students (should)collect enough mate rials before they work on this project.
上面的动词如果以名词形式出现时，后面的that从句仍然要采用虚拟语气。
He gives me the suggestion that I(should)eat breakfast every moning.
注意2：在一些惯用语之后经常需要用虚拟，来表示与事实相反或者难以实现的事情这类习语有：as if ，as though，but for，otherwise，without，wish，if，only等。
例句： But for your help，I would not have arrived here in time.
(如果没有你的帮助，我就不能准时到达)
Without your help，I would not arrive here in time.
注意3：在下列形容词引导的that从句中必须要用虚拟语气(should)do，但是由于should经常被省略，所以实际上用的就是动词原形。
这类形容词有：It is important/ necessary/ proper/ imperative/ essential/advisable等+ that
例句：It is necessary that he(should)realize his situation.
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