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
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.