连词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, both…and, not only…but also, either…or, neither…nor, (and)then等等。
20.1 并列连词与并列结构
并列连词引导两个并列的词、词组或句子。
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)例如：
Make up your mind, and you\'ll get the chance. 拿定主意，这次你会有机会的。
= If you make up your mind, you\'ll get the chance.
One more effort, and you\'ll succeed. 再努力一下，你会成功的。
= If you make one more effort, you\'ll succeed.
2) both…and 两者都。例如：
She plays (both) the piano and the guitar. 她弹钢琴，也弹吉他。
3) not only…but (also), as well as不但…而且。例如：
She plays not only the piano, but (also) the guitar. 她不但弹钢琴，也弹吉他。
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。例如
Not only does he like reading stories, but also he can even write some.他不但喜欢读小说，甚至还会写。
4) neithe…nor 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。例如：
Neither you nor he is to blame. 你和他都不该受指责。
20.2 比较and和or
1) 并列结构中，or通常用于否定句，and用于肯定句。
2) 但有时and 也可用于否定句。请注意其不同特点：
There is no air or water in the moon.
There is no air and no water on the moon.
在否定中并列结构用or 连接，但含有两个否定词的句子实际被看作是肯定结构，因此要用and。
典型例题
---I don\'t like chicken ___ fish.
---I don\'t like chicken, ___ I like fish very much.
A. and; and B. and; but C. or; but D. or;and
答案C。否定句中表并列用or, but 表转折。
判断改错：(错)We will die without air and water.
(错)We can\'t live without air or water.
(对)We will die without air or water.
(对)We can\'t live without air and water.
20.3 表示选择的并列结构
1) or意思为\"否则\"。例如：
I must work hard, or I\'ll fail in the exam. 我要努力学习，否则考试要不及格了。
2) either…or 意思为\"或者……或者……\"。注意谓语动词的主谓一致采用就近原则。例如：
Either you or I am right. 不是你对，就是我对。
20.4 表示转折或对比
1) but表示转折，while表示对比。例如：
Some people love cats, while others hate them.有些人喜欢猫，而有些人不喜欢。
典型例题
--- Would you like to come to dinner tonight?
--- I\'d like to, ___ I\'m too busy.
A. and B. so C. as D. but
答案D。but与前面形成转折，符合语意。而表并列的and， 结果的so，原因的as都不符合句意。
2) not…but… 意思为\"不是…而是…\"，后面的用词要遵循一致原则。
They were not the bones of an animal, but (the bones) of a human being. 这些不是动物的骨头，而是人的。
20.5 表原因关系
1) for是并列连词，不能置于含两个并列分句的句子的句首，只能将其放在两个分句中间。
判断改错：(错) For he is ill, he is absent today.
(对) He is absent today, for he is ill.
2) so为连词, therefore一般为副词。例如：
He hurt his leg, so he couldn\'t play in the game. 他的腿受伤了，不能上场。
I think; therefore I exist. 我思故我在。
20.6 比较so和 such
such 是形容词，修饰名词或名词词组，so是副词，只能修饰形容词或副词。so 还可与表示数量的形容词many，few，much， little连用，形成固定搭配。
so + adj. such + a(n) + n.
so + adj. + a(n) + n. such + n. (pl.)
so + adj. + n. (pl.) such +n. (pl.)
so + adj. + n. (不可数) such +n.(不可数)
so foolish such a fool
so nice a flower such a nice flower
so many/ few flowers such nice flowers
so much/ little money. such rapid progress
so many people such a lot of people
so many 已成固定搭配，a lot of 虽相当于 many，但 a lot of 为名词性的，只能用such搭配。so…that与such…that之间的转换即为 so与such之间的转换。