1．用在虛擬條件句中
虛擬條件句及其主句的謂語(yǔ)形式可歸納如下
從句謂語(yǔ) 主句謂語(yǔ)
與現(xiàn)在事實(shí)相反 were, did would (should, could, might) +v
與將來(lái)事實(shí)相反 should do, would (should, could, might) +v
（可能性極?。?nbsp; were+to+v
與過(guò)去事實(shí)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(shí)可把if省略掉，但這時(shí)要把were, had, should或could放在主語(yǔ)前面（這種結(jié)構(gòu)在口語(yǔ)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.
