您好，英语单词 if 引导的从句基本上可以分为两大类：
1）名词性从句：宾语从句，表语从句。if 通常不用于引导主语从句和同位语从句，转而使用 whether。比如：
宾语从句：He asked if I wanted to go to the cinema with him.
表语从句：This is if you go to the cinema with me.
2）状语从句：条件状语从句，通常细分6小类：
2A）零类：永远真实条件句：If you heat water to 100'C, it boils.
2B）一类：与现在事实相符条件句：If I have money, I will buy a house.
2C）二类：与现在事实相反条件句：If I had money, I would buy a house.（表示现在没有钱，所以房子未买）。二类条件句可以是虚拟一个场景，也可以是和现在事实相反。
2D）三类：与过去事实相反条件句：If I had had money, I would have bought a house.（表示过去因为没钱，所以过去没买房子，不然现在早就拥有房子了。）可以是虚拟过去的一个场景，也可以是和过去事实相反。
2E）四类：虚拟一个将来的场景条件句：If I should have enough money in 5 years, I would buy a house then.
2F）五类：错综条件句（也就是条件和主句的时间时错开来的。）：If I had had money, I would buy a house now.（条件句与过去相反，表示过去没钱；主句与现在事实相反，表示没有买房。）
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）