if条件句虚拟语气有几种,分别是什么我看新概念2册上的说有2种,一种是主句用过去将来,条件句里用过去,书上还回说了一种是在《新概念英语2》64课上说主句用 过去完成,条件句中用 would + have + 过去分词,这两种有区别吗?区别是什么?

虚拟语气在非真实条件状语从句中的用法及动词形式
① 表示与现在事实相反的情况
（条件）从句谓语动词形式 主句谓语动词形式
谓语动词用一般过去式
（be用were） should/would/could/might+动词原形（过去将来时）
eg.
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）
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）
If there were no air or water,there would be no living things on the earth.如果没有水和空气,地球上就不会有生物.(事实：地球上既有空气也有水)
If I had any money with me,I could lend you some.如果我带钱了,我就会借给你些.（事实：没带钱）
If he studied harder,he might pass the exam.如果他再努力些,就能通过考试了.（事实：学习不用功）
②表示与过去事实相反的情况
从句谓语动词形式 主句谓语动词形式
had+过去分词（过去完成时） should/would/could/might+
have+过去分词（现在完成将来时）（过去将来完成时）
eg.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）
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（可能相反或可能性很小）
从句谓语动词形式 主句谓语动词形式
If+主语+ ① should+动词
② did
③were to do
(①通常与一个表示时间状语连用)其 中were to do可能性最小,
should/would/could/might+动词原形
eg.If he would come here tomorrow,I should/would talk to him.如果他哪天来这儿的话,我就跟他谈谈.（事实：来的可能性很小）
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）
If she were to be there 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得始末.
在表示建议、命令、要求等含义的宾语从句,谓语动词常用虚拟语气,"should＋动词原形"构成,should 可省略.