because, as, for, since这几个词都是表示“原因”的连词，语气由强至弱依次为：because→since→as→for;其中because, since, as均为从属连词，引导原因状语从句；而for 是并列连词，引导并列句。
3. as是常用词，它表示的“原因”是双方已知的事实或显而易见的原因，或者理由不是很重要，含义与since相同，但语气更弱，没有since正式，常译为“由于，鉴于”。从句说明原因，主句说明结果，主从并重。例如： (1)We all like her as she is kind. 我们都喜欢她，因为她善良。
4. for用作连词时，与because相似，但它所表示的原因往往提供上文未交待过的情况。for不表示直接原因，表明附加或推断的理由，因此for被看作等立连词，它所引导的分句只能放在句子后部(或单独成为一个句子)，并且前后两个分句间的逻辑关系不一定是因果关系，其间用逗号隔开，且for不可置于句首，for的这一用法常用在书面语中，较正式。例如： It must have rained, for the ground is wet. (从“地面潮湿”作出“下过雨”的推测，但地湿并不一定是下雨所致, for不可以换为because。) The ground is wet because it has rained. (“下雨”是“地上潮湿”的直接原因。)
(4)I could not go, for / because I was ill. 我没能去，是因为我病了。
二.Because/as because 引导的是必然原因，as引导一般原因。
必然原因是指某事发生必会导致另一件事实。一般原因语气还有其因果关系不是很明显。以下两个句子请体会 The floor is wet,because it rained this morning.
As the floor is wet, it might rain this morning.(地板湿也可能是其他原因，以此看，as引导原因从句还有猜测的意味。) Because/for
Because it was wet he took a taxi．
He stole，not because he wanted the money but because he liked stealing．
—Why did you do it？ —I did it because I was angry．
He spoke in French．She was angry because he had spoken in French．
She was angry，for she didn’t know French．她生气了，因为她不懂法语。（这里用for是正确的，也可用because。）之所以有这些用法上的限定，其理由是for引导的从句不能直接说明某一特定动作发生的原因，而只能提供一些起帮助解释作用的附加说明。
例句如：The days were short，for it was now December．天短了，现在已是12月了。
He took the food eagerly，for he has eaten nothing since dawn．他狼吞虎咽地吃了起来，因为他从天亮就没吃过东西。
一、用when , while ,as soon as, not…until 填空
1. the car hit the man _______ he was crossing the road. 2. I’ll tell you the good news_______ I get there.
3. he ______ go to bed _______11o’clock last night. 4. the accident happened _______ I was on my way to work.
5. sports build the body______ reading builds the mind. 6. I used to be rather quite_______ I was young.
二、用although , though, however填空
1.they got to the airport on time______ the traffic was bad. 2. we had a wonderful_______ the food was terrible.
3. some people think winter is a bad season. _______, for me it’s a good time because I love cold, rainy weather.
三、用because, since, so… that, if填空
1.______ we’re young, we shouldn’t be too afraid of making mistakes.2.the weather was ______ nice ______ I spent the whole day in the park.3.he couldn’t come to your party______ he was ill.4.______ were you, i would wear that earrings.
1. I haven't heard from him _____ he went to America. 2. He won't be here _____ he is invited.
3. He will not go to the cinema _____ he is very busy. 4. We found the key _____ she lad left it.
5. We found the books two days ____ he had gone away. 6. We had no sooner got to the station _____ the train left.
7. He speaks english ______ he were an Englishman. 8. He is explaining clearly _______ they could understand.
9.Do not leave the room _____ you have finished the test. 10. she sang ______ she went along.
1. the children were running on the playground as fast as they can.2. since her husband had died，so she had to support her family.3. he won't go out until his mother will come.4. he was very foolish that he didn't pass such an easy exam.5. tom had gone out as soon as his mother got home.6. it was three months since he came to our school.7. the playground of our school is larger than their school.8. the streets in nanjing are wider than shanghai.9. whatever there is plenty of sun and rain，the fields are green.10. she sings songs as if she is a bird.
定语从句的基本结构为：先行词 + 关系代/副词 + 句子 考点1. 关系代词: that, which, who, whom, whose 关系副词: when, where, why.
There are lots of things ________ I need to prepare before the trip. A. who B. that C. whom D. whose
This is the dictionary ________ Mum gave me for my birthday. A. which B. what C. whose D. whom
Sorry, we don’t have the coat ________ you need. A. what B. who C. whom D. which
Jamie is a young cook ________ wants to improve school dinners.
A. who B. whose C. whom D. which d. whose 作定语,表示所属关系。
I like the teacher ________ classes are very interesting and creative. A. which B. who C. what D. whose
e. when 作时间状语:When =介词 + which
She will never forget the day when (=on which) she was married.
f. where 作地点状语 where = 介词 + which
I want to know the place where (= in which) I was born.
g. why 作原因状语, 其先行词常常是the reason. why = for which
That is the reason why(= for which) I’m late.
1. I prefer shoes that are cool.
2. I like a pizza that is really delicious.
3. 有时只能用 that , 不用 which, 常见的情况有六种：
1).当先行词是 all, any, few, little, none, anything, everything, nothing ,或被它们修饰时。
That’s all that I know. Is there anything that I can do for you?
2).当先行词被形容词最高级或序数词修饰That is the most interesting book that I have ever read.
3).当先行词有the very, the only, the same等修饰时。
That’s the only thing that I can do now. These are the very words that he used.
4).当主句以 who 或 which开头时，定语从句的关系词用 that, 而不用 which 或 who.
Who is the girl that spoke to you just now? Which is the pen that you lost ?
The man and his dog that I always meet are standing by the gate.
6).当关系代词在从句中作表语时。China is not the country that it was.
4. 只用 which ,不用that 的情况：1).关系代词前有介词时。This is the cage that / which Polly lives in.
= This is the cage in which(where) Polly lives . 2).先行词本身为 that. The clock is that which tells the time.
3). 在非限制性定语从句中。His English, which used to be poor , is excellent now.