连词是一种在句子与句子之间，短语之间以及名词等其他词语之间起连接作用的
虚词，它不能单独作句子的成份。按其意义可分为并列连词和
从属连词两大类。�并列连词连接的双方是对等的。常有的并列连词有and, both…and, either…or, neither…nor， not only…but also, as well as等。但如果连接的两部分意义不趋向一致，意义有转折的并列连词有：but, however, while (而)，only (只不过)。还有表示选择关系的并列连词，如：or,
or else, otherwise… 再有的是连接双方，互为因果，或表示前因后果的连词有：for, so, therefore (因此)，then等。�从属连词在初中范围内常常用来连接
名词性从句，如：that, if, whether, 其次用来连接状语从句。其中有
原因状语从句，常用的连接词有：when while, as, since, before, after, once,
as soon as, until, till 连接条件状语的连词有：if, unless, as long as 等，而原因状语的连接词有because, since, as, now that (既然)。目的、结果、方式、比较、地点等状语从句的连接词有：so that, so…that, such…that, as…as, than, where… 它们在句子与文章中几乎无处不见。�具体用法见下表。�
连词用法一览表
种类 功用 例句
并列连词 连接具有并列关系的 词 He knows neither English nor French.
短语 Are you going by bus or on foot?
分句 Mary was a good girl, but she had one shortcoming.
从属连词 引导： 状语从句 I'll do it as you told me.
You will be late unless you hurry.
连接代词和连接副词 主语从句 What he said proved true.
When we'll start has not been decided yet.
表语从句 This is why he didn't come yesterday.
That is where he lives.
宾语从句 The man asked me which I liked best.�
I can't understand why she is so late.
关系代词和关系副词 定语从句 Nicotine is a drug that gets one into the habit of smoking.�
He came last night when I was out.