1、表示与现在事实相反的情况
从句：If+主语+动词一般过去时(Be动词用were)主句：主语+ should/would/might/could+do：If I were you，I would take an umbrella.如果我是你，我会带把伞。（事实：我不可能是你）
2、表示与过去事实相反的情况
从句：If+主语+had+done主句：主语+should/would/might/could+have done：If I had got there earlier，I should/would have met her.如果我早到那儿，我就会见到她。（事实：去晚了）
3、表示与将来事实相反
从句：①if+主语+were to do
②if+主语+should+do
③if+主语+动词一般过去式（be动词用were）主句:主语+should/would/might/could+do:If he should come here tomorrow，I would talk to him.如果他明天来这儿的话，我就跟他谈谈。（事实：来的可能性很小，不可能）