语法小点——虚拟语气(2)

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）