在句子中做条件状语的从句称为条件状语从句.可置于句首、句尾,有时还可置于主语和谓语之间.引导条件状语从句的连接词有：if ,unless,as/so long as等
1.if引导的条件状语从句
表示在某种条件下某事很可能发生.
If we pay much attention to the earth,we'll have a better world.
如果我们多关爱地球,就会拥有一个更美好的世界
If it rains tomorrow,what shall we do?
如果明天下雨,我们怎么办?
注意：if还可以引导虚拟条件句,表示不可实现的条件,从句中多用一般过去时或过去完成时.
What would happen if there were no water
如果没有水会怎样?
2.unless引导的条件状语从句
unless的意思是如果不,除非.可与if...not互换.
You will fail in English unless you work hard.
你英语考试会不及格的,除非你更加努力.
3.as /so long as引导的条件状语从句
as /so long as意思是“只要”
So long as you're happy,i will be happy.
只要你高兴,我就高兴.
4.once引导的条件状语从句
once引导的条件状语从句,意为“一旦...,就”,从句可置于主句前或后
Once you begin,never stop.
一旦开始,就绝不要停止
Maths is easy to learn once you understand the rules.
一旦你理解了规则,数学就不难学了.
提示：在条件状语从句中,用一般现在时代替一般将来时,用一般过去时代替过去将来时.
If the weather is bad next week,we'll put off the climbing.
如果下周天气不好,我们将推迟爬山.
Father told me if i worked hard he would buy me a gift.
爸爸告诉我,如果我努力学习,他会给我买一份礼物.