a.与现在事实相反的假设
条件从句 主句
一般过去时（be用were） should（would）等 +动词原形
例如：If they were here,they would help you.如果他们在这儿,
含义：They are not here,they can’t help you.
b.与过去事实相反的假设
条件从句 主句
过去完成时 should（would）等+ have+ 过去分词
例如：If he had come yesterday,I should / would have told him about it.如果他昨天来的话,我会把这件事告诉他的.
含义：He did not come yesterday,so I did not tell him about it.
c.表示对将来不大可能发生的事情的假想
条件从句 主句
一般过去时 should/would等 + 动词原形
were+ 不定式
should+ 动词原形
例如：If you succeeded,everything would be all right.如果你将来成功了,一切都会好的.
If you should succeed,everything would be all right.
If you were to succeed,everything would be all right.
含义：You are not likely to succeed,everything will be what it is now.