英语中，为了避免不必要的重复，经常用so, not, to, do, does 代替前面出现的动词或相关内容。

(1) 从上两例中看出，do / does / did 代替动词。

He translated the article better than I did.   （did代替wrote it）

— Do you think she is clever?

— I think so.   （so代替 she is clever）

(2) "so 与 not" 代替某个词、短语、句子等，通常用于hope, think, believe, expect, suppose, be afraid, fear, imagine, etc后作宾语。

— Is it correct?

— I'm afraid not.  (not correct)

(3) "to" 用作不定式，常跟随下列动词：want, mean, hope, expect, refuse, seem, intend, be, afraid, etc.

 I asked him to go to the party, but he refused to. (go to the party)

(4) "do so, do that, do it" 用来代替动态动词，而不能代静态动词。

— He gave up studying English.

— Why did he do so?   (= give up studying English)

— The dish tastes nice.

— Yes, so it does.  (tastes nice)

（此句不能用it does it 或 it does so, 因taste属静态动词。）

(5) 为使语言精炼，避免不必要的重复，对话中常用省略形式。

— He is thinking of buying a car?

— Is he? （这里，“thinking of buying a car”被省略了。）

— Will he come back in time?

— Perhaps. （省略了he will come back in time.）