主从复合句--状语从句状语从句：在复合句中作状语,其位置可以在主句前或主句后。
状语从句可以分为时间、地点、原因、目的、结果、比较、让步、条件等几种。
状语从句由从属连接词引导。
时间状语从句通常由when ,as ,while, after, before, since, as soon as ,since, till (until), while, whenever等引导。
时间状语从句一般放在句首或句尾,特别注意,时间状语从句不允许使用将来时,而应该用现在时替代。
如：When you finish the work, you maygo out to play with Sam. (你完成工作就可以出去和Jim一起玩了。
) / I won’t leaveuntil Mum comes back. (妈妈回来了我才会走。
)地点状语从句通常由where, wherever等引导。
如：Go back where you came from! (哪里来还滚到哪里去！) / I will never forget to catch the thief who stole my necklacewherever he may be. (我永远也不会忘记去抓住那个偷我项链的贼,无论他会在哪里。
)原因状语从句通常由because, since, as等引导,一般放在句首或句尾。
如：He went abroad because his father had found a good university for him. (他出国了,因为他父亲给他找了一所好大学。
)目的状语从句通常由so that..., so...hat..., in order that...等引导,往往放在句尾,从句中通常含有can / could / may / might等情态动词。
如：He got up earlier so that he couldcatch the first train. (他起身更早为的是赶上第一班车。
)结果状语从句通常由so that..., so...that... 等引导,放在句尾。
结果状语从句一般表示已经发生的事情,故多为过去时态。
如：He lost so many bikes that he decided never tobuy a new one. (他丢了那么多辆自行车,他决定再也不买新车了。
)比较状语从句通常由as, than, as (so)...as等引导,一般省略从句的谓语部分,只剩下名词或代词(用主格或宾格均可)。
如：Jane is much taller than I/me. (Jane比我高多了。
)/ I don’t ha ve as many books as you (do). (我书没有你多)让步状语从句通常由though (although), as, even if( even though), however, whatever等引导。
如：Even if you pay the debt(债务) for me, I will not thank you because it hasnothing to do with me. (即使你替我还了债我也不会感谢你,因为它与我毫无关系。
)/ He wears a T-shirt though it is very cold. (他穿了一件T恤衫,尽管天很冷。
)条件状语从句通常由if, unless, as long as等引导,条件状语从句一般放在句首或句尾,特别注意,时间状语从句不允许使用将来时,而应该用现在时替代。
You will certainlyfail in the coming final exams unless you work much harder. (即将来到的期末考试你肯定考不及格,除非你更用功。
) / If it doesn’ t rai n tomorrow, we shall go hiking.(如果明天不下雨我们就要去徒步旅行。
)[注意] 1、because与so；(al)though与but；if与so不可以同在一个句子中成对出现。
用来修饰主句中的动词，副词和形容词的从句叫状语从句。
根据其含义状语从句可分为时间状语从句，地点状语从句，条件状语从句，原因状语从句，结果状语从句，比较状语从句，目的状语从句，让步状语从句。
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 b elieve 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.比较while, when, as1）as, when 引导短暂性动作的动词。
例如：Just as / Just when / When I stopped my car, a man came up to me. 我一刹车，有一个人向我走来。
2）当从句的动作发生于主句动作之前，只能用when 引导这个从句，不可用as 或while。
例如：When you have finished your work, you may have a rest. 赶完活后，你可以休息一下。
3）从句表示"随时间推移"连词能用as，不用when 或while。
例如：As the day went on, the weather got worse.日子一天天过去，天气越变越坏。
比较until和till此两个连词意义相同。
肯定形式表示的意思是"做某事直至某时"，动词必须是延续性的。
否定形式表达的意思是"直至某时才做某事"。
动词为延续性或非延续性都可以。
正确使用这两个连词的关键之一就在于判断句中的动词该用肯定式还是否定式。
肯定句：I slept until midnight. 我一直睡到半夜时醒了。
Wait till I call you. 等着我叫你。
（在肯定句中可用before代替。
例如Let's get in the wheat before the sun sets.）否定句：She didn't arrive until 6 o'clock. 她直到6点才到。
Don't get off the bus until it has stopped. 公共汽车停稳后再下车。
I didn't manage to do it until you had explained how. 直到你教我后，我才会做。
区别：1）until可用于句首，而till通常不用于句首。
例如：Until you told me, I had heard nothing of what happened.直到你告诉我以前，出了什么事我一点也不知道。
2）until when 疑问句中，until要放在句首。
例如：---Until when are you staying?你呆到什么时候？---Until next Monday.呆到下周一。
注意：否定句可用另外两种句式表示。
1）Not until…在句首，主句用倒装。
例如：Not until the early years of the 19th century did man know what heat is.直到19 世纪初，人类才知道热为何物。
Not until I began to work did I realize how much time I had wasted.直到工作，才认识到我已蹉跎了几多岁月。
2）It is not until… that… 。
例如：It was not until I began to work that I realized how much time I had wasted.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. 原因状语从句比较because, since, as和for：1）because语势最强，用来说明人所不知的原因，回答why提出的问题。