1. because, since, for
（1）because是从属连词，意为“因为”，表示造成某种情况的直接原因，语气最强，常用来回答why的提问。
-Why didn't you come to my birthday party? 你为什么没来参加我的生日聚会？
-because I was too busy.因为我太忙了。 （2）since, as往往表示众所周知的原因，常译为“既然，由于”。since从句通常位于主句之前，其语气比as稍强；as 引导的从句位于主句前后均可。
Since you come, wait for more times please.既然来了，就多等一会吧。
As he has no car, he can't there easily.由于他没车，所以不能顺利到达那里。 （3）for 是并列连词，语气最弱，只对前面的分句加以解释或表示推断的原因，并多用于书面语中。for所连接的分句只能用于句尾，前面用逗号与另一分句隔开。
The days is very short, for it is now December.白天很短，因为已经十二月了。 2.whether, if
用作从属连词时，whether和if都意为“是否”，一般情况下，它们都可以互换，口语中常用if，但在以下情况下，只能用whether.
（1）引导主语从句置于句首时
Whether it is true remains a question.它是真是假还是个问题。 （2）引导表语从句时
The question is whether it will rain.问题是会不会下雨。 （3）引导同位语从句时
There is a doubt he is fit for the job.他是否胜任这个工作还是个疑问。 （4）与or not 连用时
Let me know whether or not you can come.请让我知道你是否能来。 （5）与动词不定式连用时
I couldn't decide whether to do or not.我不能决定是否做那事。 （6）引导介词的宾语从句时
There has been no news about whether they have finished their work.仍然没有他们是否完工的消息。 3.when,while ,as
when ,while, as作从属连词，都引导时间状语从句，表示主句动作与从句动作同时发生
（1）when引导的从句可以用延续性动词，但也可以用短暂性动词，而while, as引导的从句只能用延续性动词。
When you knocked at the door, I was having a bath.你敲门时我正在洗澡。
It began to rain while we were walking in the park.我们在公园散步时，天开始下雨了。
The students took notes as they listened.学生们边听课边做笔记。 （2）while 和as引导的从句的谓语动作经常与主句谓语动作同时发生，而when引导的从句的谓语动作可以发生在
主句的动作之前，之后，也可以同时发生
When/While/As we were dancing, a stranger came in.我们正跳舞时，一个陌生人走进来了。
When I got to the station, the train had left.我赶到车站时，火车已经开走了。
When he had finished his homework, he took a short rest.写完作业后，他休息了一会。 （3）强调主，从句动作同时进行时，从句的时间概念淡化，而主要表示主句动作发生的背景或条件时只能用as。
As years go by, China is getting stronger and richer.随着时间一年年过去，中国变得越来越富强。
As the sun rose, the fog disappeared.太阳一出来，雾随之消散。 考试热点——考查连词的用法
1. Tom was about to close the window _____his attention was caught by a bird.(2010.全国)
A.when B.if C.and D.till
2.-I wonder how much you charge for your services.
-The first two are free____the thirs costs $30.(2009.安徽)
A.while B.until C.when D.before
3.John thinks it won't be long ____he is ready for his new job.(2010.安徽）
A.when B.after C.before D.since KEYS
1.A
【解】be about to do sth when..."正要做某事时……”，为固定结构，when在此用作并列连词。if“如果”引导条件状语从句；and 连接顺接关系的并列句；till“直到”引导时间状语从句。
2.A
【解】while 在此表示对比意义，符合题意。until“直到”；when“当……时候”；before“在……之前”。
3.A
【解】"It will not be +时间段+before..."表示 “用不了多长时间就……”，是固定句式。when“当……时候”；after“在……之后”；since“自从……”，常用于“It is /has been+时间段+since...”句式。
1.看中文写英文
暂时的_____ 当代的_______ 体贴________ 合同________
2.翻译句子
初来乍到，应该学会适应环境。
One should ____________________ when he moves to a new place.
她太挑剔了，总是找别人毛病。
She is picky and always ___________ everyone.
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