英语虚拟语气倒装问题!were,had,should这三个用于倒装时有什么区别啊?

1当与过去相反时 去掉if 把had前置,Had you come to the party,you would have met ZhouBiChang
2当与现在相反时 去掉if 把were前置：Were I you ,I would go back
3当与将来相反时 去掉if 把should前置Should it rain ,the crop should be saved