3. 复合句：由主句和从句构成，从句由从属连词引导。从句包括：状语从句，名词性从句和定语从句。简单句 + 从属连词 + 简单句 = 复合句。
1) 状语从句：充当状语的句子。状语从句和主句互相独立，既不互相包含也不交叉，通过不同的从属连词连接起来。在含有状语从句的复合句中，主句是个完整的句子。状语从句一般在主句后，也可在主句前，表强调，并且使用“，”隔开。
状语从句包括：时间状语从句、条件状语从句、原因状语从句，目的状语从句和结果状语从句等。
①时间状语从句，引导词有：after、as、as soon as、before、by the time、each time、since、till、until、when、while等。
A. 一般过去时态的状语从句。
如：It was raining when we arrived.
我们到达的时候，天正下着雨。
B.某些时间状语从句的复合句中，符合主将从现原则，从句通常用现在时代替将来时。
如：Whenyou read the poem a second time, the meaning will becomeclearer to you.
当你再读一遍这首诗，你就会更清楚它的含义。
C.while引导的时间状语从句可用作进行时态。
如：Whilethe twins were sweeping the floor, the teacher came in.
= The twins were sweeping the floorwhen the teacher came in.
老师进来时，这对双胞胎正在扫地。
D.现在完成时态的时间状语从句：since + 过去时态的从句。
如：Three months have passedsinceI arrived in Sydney. 我到悉尼已经三个月了。
F.过去完成时态的时间状语从句：by the time到……之前；before；when。
如：The train had already leftwhen they got to the station.
当他们到达车站时，火车已经开走了。
By the time/ Before I arrived, Kate had just left.
在我到达之前，凯特刚刚离开。
G. After引导的时间状语从句可以使用过去完成时态。
如：Afterhe had finished the homework, he went home. 他完成作业后才回家。
②条件状语从句，引导词有：if 如果；unless 除非，如果不；once 一旦……就……；as long as 只要。在含有条件状语从句的复合句中，符合主将从现原则，从句通常用现在时代替将来时。
如：We will go fishingif it doesn't rain tomorrow.
= We will go fishingunless it rains tomorrow.
如果明天不下雨，我们就去钓鱼。
You'll like her once you get to know her.
你一旦了解了她就会喜欢她的。
You may borrow my book as long as you keep it clean.
只要你保持书的清洁，你就可以把我的书借去。
③目的状语从句。从句中谓语动词常与can，could，may，might等情态动词连用。引导词有：so that 以便于、为了；in order that 为了。
如：I stood the little girl on a chair so that she could see.
我让小女孩站到椅子上，好让她看得见。
She left early in order that she wouldn't miss the train.
她早早出发为了不错过火车。
④结果状语从句一般放在主句之后。引导词有：so...that... 那么的……以至于……；such...that... 如此……以至于……。
如：He has to earn so lots of money thathe can raise his big family.
他不得不赚这么多钱才能养活他的大家庭。
It was such fine weather yesterday that we went swimming.
昨天天气很好，我们去游泳了。
