连词是一种虚词，它在句中主要起辅助作用，用来连接词与词、短语与短语或句子与句子。根据其在句中所起的语法作用，连词主要分为并列连词和从属连词。
（一）并列连词
并列连词是用来连接具有并列关系的词、短语或句子。
（1）表示并列成分的连词有and，both．．．and，neither．．．nor．．．，not only．．．but also．．．等。and表示并列与对称关系；both．．．and．．．意为“两者都……”；neither．．．nor．．．意为“既不……也不……”；not only．．．but also．．．意为“不仅……而且……”。neither．．．nor．．．和not only．．．but also．．．连接的词语作主语时，谓语动词的单复数形式采用就近原则。例如：
Please come and have dinner with us．
Neither you nor Li Hua has been to the GREat Wall before．
The ground must be just right neither too wet nor too dry．
（2）表示选择关系的连词有or，either．．．or等。or表示选择其一或“否则，要不然”。either．．．or．．．意为“要么……要么……；不是……就是……”。例如：
Hurry up，or you won't catch the early bus．
You may either stay here or go with us．
（3）表示意义转折的连词有but，while，yet等。例如：
The car was old，yet it was in excellent condition．
He is listening to the music while he is washing clothes．
—Would you like to come to dinner tonight？
—I'd like to，but I'm too busy．
（4）表示因果关系的连词有for，so。例如：
That was our first lesson，so she didn't know all our names．
I must go now，for my sister is waiting for me．
（二）从属连词
从属连词用来引导从句。主要分为两类：一种是引导状语从句，另一种是引导宾语从句、表语从句等。（详见状语从句和宾语从句）
（1）状语从句
引导时间状语从句的从属连词有when，after，before，while，as，until，not．．．until．．．，as soon as，since等。例如：
Mr Brown had already left when I got to his office．
I won't go home until I finish my work．
（2）引导宾语从句的连词：that，if，whether，what，who，which，whose，when，where，why等。例如：
Alice wanted to know if her grandmother liked the bag．
Can you tell me where the Red Star Hospital is？
I don't know whether he still lives here after so many years．
I don't know why Jane was late for school this morning．
I don't know what I can do for you．
