1、表示与现在事实相反的情况：
从句：主语+过去时
主句：主语+should/would/could/might+do
eg：
1．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．If I knew his telephone number,I would tell you. 如果我知道他的电话号码,我就会告诉你.（事实：不知道）
2、表示与过去事实相反的情况
从句：主语+had+done
主句：主语+should/would/could/might+have done
eg：
1． If I had got there earlier,I should/could have met her.如果我早到那儿,我就会见到她. （事实：去晚了）
2．If he had taken my advice,he would not have made such a mistake.如果他听我的劝告的话,就不会犯这样的错误了. （事实：没有听我的话）
3、表示对将来情况的主观推测
从句：主句
①if+主语+were to do ：①主语+should/would/could/might+do
②if+主语+did/were ②主语+should/would/could/might+do
③if+主语+should+do ③主语+should/would/could/might+do
eg：
1．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）
2．If there were a heavy snow next Sunday,we would not go skating. 如果下周日下大雪,我们就不能去滑冰了.（事实：不知能否下雪）
3．If she were to be here next Monday,I would tell her about the matter. 如果她下周一来这儿的话,我就会告诉她这件事的始末.
