连词是一种虚词， 它不能独立担任句子成分而只起连接词与词，短语与短语以及句与句的作用。连词主要可分为两类：并列连词和从属连词。并列连词用来连接平行的词、词组和分句。如：and, but, or, nor, so, therefore, yet, however, for, hence, as well as, bothand, not onlybut also, eitheror, neithernor, （and）then等等。
并列连词引导两个并列的句子。
1） and 与or
判断改错：
（错） They sat down and talk about something.
（错） They started to dance and sang.
（错） I saw two men sitting behind and whisper there.
（对） They sat down and talked about something.
（对） They started to dance and sing.
（对）I saw two men sitting behind and whispering there.
解析：
第一句： and 连接两个并列的谓语，所以 talk 应改为 talked.
第二句：and 连接两个并列的动词不定式，第二个不定式往往省略to，因此sang 应改为 sing.
第三句：and 连接感观动词saw 后面的用作的宾补的两个并列分词结构，因此whisper应改为whispering.
注意：and 还可以和祈使句或名词词组连用表示条件。（or也有此用法）
Make up your mind, and youll get the chance.
= If you make up your mind, youll get the chance.
One more effort, and youll succeed.
= If you make one more effort, youll succeed.
2） both and 两者都
She plays （both） the piano and the guitar.
3） not onlybut （also）, as well as不但而且）
She plays not only the piano, but （also） the guitar.
注意： not only but also 关联两个分句时，一个分句因有否定词not 而必须倒装。
Not only does he like reading stories, but also he can even write some.
4） neithenor 意思为既不也不谓语动词采用就近原则，与nor后的词保持一致。
Neither you nor he is to blame.