1. 时间状语从句
(1)时间状语从句常用when, as, while, before, after,since, till, until, as soon as等连词来引导。例如：
It was raining hard when got to school yesterday.
While he was doing his homework, the telephone rang.
As he walked along the lake, he sang happily.
He had learned a little Chinese before he came to China.
After he finished middle school, he went to work in a factory.
(2)在时间状语从句里，通常不用将来时态，用现在时态表示将来的动作或状态。例如：
I’ll ring you up as soon as I get to New York.
I will tell him everything when he comes back.
He won’t believe it until he sees it with his own eyes.
(3)在带有till或until引导的时间状语从句的主从复合句里，如果主句用肯定式，其含义是“一直到……时”，谓语动词只能用延续性动词。如果主句用否定式，其含义是“直到……才……”, “在……以前不……”, 谓语动词可用瞬间动词。例如：
The young man read till the light went out.
Let’s wait until the rain stops.
We won’t start until Bob comes.
Don’t get off until the bus stops.
2. 条件状语从句
(1)条件状语从句通常由if, unless引导。例如：
What shall we do if it snows tomorrow?
Don’t leave the building unless I tell you to.
(2)在条件状语从句里，谓语动词通常用现在时态表示将来的动作或状态。例如：
I’ll help you with your English if am free tomorrow.
He won’t be late unless he is ill.
(3)“祈使句 + and (or)+ 陈述句” 在意思上相当于一个带有条件状语从句的复合句。例如：
Hurry up, or you’ll be late.
=If you don’t hurry up, you’ll be late.
Study hard and you will pass the exam.
=If you study hard, you will pass the exam.
3. 原因状语从句
(1)原因状语从句通常由because, since, as引导。例如：
He didn’t come to school because he was ill.
As it is raining, we shall not go the zoo.
Since you can’t answer the question, I’ll ask someone else.
(2)because表示直接原因，语气最强。Because引导的原因状语从句多放在主句之后。回答由why提出的问题，只能用because。As和since语气较弱，一般用来表示明显的原因。由as和since引导的原因状语从居多放在句首。例如：
------Why aren’t going there?
------Because I don’t want to.
As he has no car, he can’t get there easily.
Since we have no money, we can’t buy it.
(3)because和so不能同用在一个句子里。
