7. 常用连词的用法辨析
（1）while, when, as
这三个连词都可引导时间状语从句，但用法有所不同。
1) 当某事正在进行的时候，又发生了另一件事。While, when, as 都可用来引导表示“背景”的时间状语从句。例如：
As/When/While I was walking down the street I noticed a police car.
2) 当两个长动作同时进行的时候，最常用的是while。例如：
While mother was cooking lunch, I was doing my homework.
3) 当两个动作都表示发展变化的情况时，最常用的是as。例如：
As children get older, they become more and more interested in things around them.
4) 当两个短动作同时发生时，或表示“一边…一边…”时，最常用as。例如：
Just as he caught the fly, he gave a loud cry.
She looked behind from time to time as she went
5) 当从句的动作先于主句的动作时，通常用when。例如：
When he finished his work, he took a short rest.
6) 当从句是瞬间动作，主句是延续性动作时，通常用when。例如：
When John arrived I was cooking lunch.
（2）as, because, since , for
这四个词都可表原因，但用法有区别。
1) 如果原因构成句子的最主要部分，一般用because。因此，because引导的从句往往放在句末。例如：
I stayed at home because it rained.
---Why aren’t you going?
---Because I don’t want to.
2) 如果原因已被人们所知，或不如句子的其它部分重要，就用as或since。Since比as稍微正式一点。As和since 引导的从句一般放在句子的开头。例如：
As he wasn’t ready, we left without him.
Since I have no money, I can’t buy any food.
3) for用来补充说明一种理由，因此，for引导的从句几乎可以放在括号里。For引导的句子不放在句子的开头。例如：
I decided to stop and have lunch----for I was feeling quite hungry.