三、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.

四、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.
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 ……"
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.

五、表原因关系
1) for
判断改错：
( 错 ) For he is ill, he is absent today.
( 对 ) He is absent today, for he is ill.
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。
2) so, therefore
He hurt his leg, so he couldn't play in the game.
注意：
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.

六、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。
so…that 与 such…that 之间的转换既为 so 与 such 之间的转换。