当非真实条件句中含有had, should或were时,可以去掉if,而把had, should或were提到句首构成倒装.如：
If I had know his telephone number yesterday, I would have called him up.
--> Had I know hs telephone number yesterday, I would have called him up.
句意：要是我昨天知道他的电话号码的话,我就会打电话给他了.
If I were you, I would make the similar mistake.
--> Were I you, I would make the similar mistake.
If it should rain tomorrow, we would cancel our hiking in the mountains.
--> Should it rain tomorrow, we would cancel our hiking in the mountains.
