一、连词属于虚词之一，它不能在句子里独立地担任句子成分，而只能起连接作用--连接词与词、短语与短语、句子与句子。例如：
Would you like some tea or coffee? （连接词与词）您是喝茶，还是喝咖啡？
Do we have our meeting in the morning or in the afternoon?
（连接短语与短语） 我们是上午开会，还是下午开会？
Do it carefully, or you'll make some mistake.
（连接句子与句子）认真地干，不然会出差错的。
二、连词主要可分成两类：
并列连词（Coordinating Conjunctions）
并列连词用来连接平行对等（即互不从属）的词、词组、或分句。按其表示的不同意思有下面几种：
1）表示转折意思 but , yet , however , nevertheless
2）表示因果关系 for , so , therefore , hence
3）表示并列关系 and , or , either…or , neither…nor , not only…but (also) , both…and , as well as
从属连词（Subordinating Conjunctions）
从属连词是用来引起从句的，它在主句和从句中间起连接作用。具体地说，从属连词主要引起名词从句（主语从句、宾语从句、表语从句、同位语从句）和状语从句，不会引起定语从句。
| 引起名词从句的连词： | that , whether , if |
|
引起状语从句的连词 | |
| 时间状语从句： | when , whenever , as , while, before , after, until, till, since, ever since, once, as soon as, every time, the moment, instantly, directly |
| 地点状语从句： | where , wherever |
| 原因状语从句： | as, because, since, seeing(that) , considering (that) , now that , not that…but that |
| 目的状语从句： | that , so that , in order that , lest , for fear that , in case |
| 结果状语从句： | that , so … that , such … that |
| 条件状语从句： | if , unless , suppose , supposing ,so/as long as, so far as, on condition(that ) ,provided ( that ) |
| 让步状语从句： | though , although , no matter , even if , even though , however , whatever |
| 比较状语从句： | than , as |
| 方式状语从句： | as , as if , as though |
| 问天词典 |
