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。）
— 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.）