1．用在虚拟条件句中

    虚拟条件句及其主句的谓语形式可归纳如下

                 从句谓语              主句谓语

与现在事实相反  were, did   would (should, could, might) +v

与将来事实相反  should do,  would (should, could, might) +v

（可能性极小）  were+to+v  

与过去事实相反  had done    would (should, could, might) + have done

    例1：I would certainly go if I had the time.

    例2：How nice it would be if you could stay a bit longer.

    例3：She would have come if we had invited her.

    如果条件从句中包含有were, had, should或could，有时可把if省略掉，但这时要把were, had, should或could放在主语前面（这种结构在口语中较少使用）。例如：

    1）Had we made (= If we had made ) adequate preparations, we might have succeeded.

    2）Were there no gravity, there would be no air around us.

    3）Should it rain tomorrow, the meeting would be put off.