but在虚拟语气中的用法but的虚拟语气用法.请举例.

跳层虚拟句（即句子一半为虚拟,另一半为陈述语气；but前句用虚拟而or/otherwise后句用虚拟） (but)
He would put on weight,but he doesn't eat much.(与现在事实相反)
He would have put on weight,but he didn't eat much.(与过去事实相反)
(or,or else,otherwise)
I forget where I read the article,or I would show it to you now.(与现在事实相反)
Mary couldn't have received my letter,otherwise she would have replied before now.(与过去事实相反)