與現在事實相反 were, did would (should, could, might) +v
與將來事實相反 should do, would (should, could, might) +v
與過去事實相反 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.
如果條件從句中包含有were, had, should或could，有時可把if省略掉，但這時要把were, had, should或could放在主語前面（這種結構在口語中較少使用）。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.