6.从属连词
从属连词用来引导从句。常见的从属连词有：
(1)引导时间状语从句的after, before, when, while, as, until, till, since, as soon as等。(2)引导条件状语从句的if, unless等。
(3)引导原因状语从句的because, as, since等。
(4)引导目的状语从句的so that, in order that等。
(5)引导让步状语从句的though, although, even if等。
(6)引导结果状语从句的so that, so…that, such…that等。
(7)引导比较状语从句的than, as…as等。
(8)引导名词从句的that, if , whether等。
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.www.zhongkao5.com//
(3)if, whether
if和whether都可作“是否”讲，在引导宾与从句是一般可互换。例如：
I wonder whether (if) you still study in that school.
I don’t know whether (if) he likes that film.
在下列情况下，只能用whether,不能用if：
1)引导主语从句时。例如：
Whether he will come to the party is unknown.
2)引导表语从句时。例如：
The question is whether I can pass the exam.
3)在不定式前。例如：
I haven’t made up my mind whether to go there or not.
(4)so…that, such…that
1)so…that中的so是个副词，其后只能跟形容词或副词，而such…that中的such是个形容词，后接名词或名词短语。例如：
I’m so tired that I can’t walk any farther.
It was such a warm day that he went swimming.
2)如果在名词之前有many, much, little, few时，用so,不用such。例如：
He has so little education that he is unable to get a job.
I have had so many falls that I am black and blue all over.
(5)either…or…， neither…nor, not only…but also…
这三个连词词组都可连接两个并列成分。当它们连接两个并列主语时，
谓语动词要随相邻的主语变化。例如：
Either you or he is wrong.
Neither he nor his children like fish.
Not only the teacher but also the students want to buy the book.
(6)although, but
这两个连词不能用在同一个句子中。例如：我们不能说“Although he is
over sixty, but he works as hard as others.”这个句子应改为：Although he is over sixty, he works as hard as others.或He is over sixty, but he works as hard as others.
(7)because, so
这两个连词同样不能用在同一个句子中。例如：我们不能说“Because John
was ill, so I took him to the doctor.” 这个句子应改为Because John was ill, I took him to the doctor.或John was ill, so I took him to the doctor.