连词是用来连接词与词、词组与词组或句子与句子、表示某种逻辑关系的虚词。连词可以表并列、承接、转折、因果、选择、假设、比较、让步等关系。在中考即将来临之前，我们一起来复习一下常见连词的用法吧！
一、 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.
二、 because/ as/ 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.
三、 if/ 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.
四、 so/ 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.
五、 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.
六、 although/ but这两个连词不能用在同一个句子中。例如：
Although he is over sixty, but he works as hard as others.（错误）
Although he is over sixty, he works as hard as others. 或 He is over sixty, but he works as hard as others.（正确）
七、 because/ so这两个连词同样不能用在同一个句子中。例如：
Because John was ill, so I took him to the doctor.（错误）
Because John was ill, I took him to the doctor. 或 John was ill, so I took him to the doctor.（正确）
