20. 连词连词是一种虚词， 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, both…and, not only…but also, either…or, neither…nor, (and)then等等。
1) and 与or
(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.
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。
Make up your mind, and you'll get the chance.
= If you make up your mind, you'll get the chance.
One more effort, and you'll succeed.
= If you make one more effort, you'll succeed.
2) both …and 两者都
She plays (both) the piano and the guitar.
3) not only…but (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) 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。
Neither you nor he is to blame.
20.2 比较and和or1) 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.
---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。
(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"。
I must work hard, or I'll fail in the exam.
2) either…or 意思为"或者……或者……"。注意谓语动词采用就近原则。
Either you or I am right.
20.4 表示转折或对比1) but表示转折，while表示对比。
Some people love cats, while others hate them.
--- Would you like to come to dinner tonight?
--- I'd like to, ___ I'm too busy.
A. and B. so C. as D. but
2) not…but… 意思为"不是……而是……"
not 和but 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.
20.5 表原因关系1) for
(错) For he is ill, he is absent today.
(对) He is absent today, for he is ill.
2) so, therefore
He hurt his leg, so he couldn't play in the game.
a. 两个并列连词不能连用，但therefore, then, yet.可以和并列连词连用。
You can watch TV, and or you can go to bed.
He hurt his leg, and so / and therefore he couldn't play in the game.
b. although… yet…，但although不与 but连用。
(错) Although he was weak, but he tried his best to do the work..
(对) Although he was weak, yet he tried his best to do the work.
20.6 比较so和 such其规律由so与such的不同词性决定。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。