2．由do，so等组成的复合替代形式
（1）“so +助动词+主语”。例如：
 “Mary will enter the university in September.” “So will Joan.”
（2）“so +主语+助动词”。例如：
 “I’ve been to Beijing.” ”So you have.”
注：这种结构中第二句的主语与第一句的主语是同一个人，否则应像“so +助动词+主语”的结构一样，主谓要倒装。
（3）“主语+（助动词+）do + so”。例如：
 “Have you sent your plan to the committee?”
 “I did so yesterday.”
（4）“主语+（助动词+）do + that”。例如：
 “Do you know who broke the television set?”
 “I heard John did that.”
（5）“主语+（助动词+）do + it”。例如：
“My brother said he was going to send a letter of protest to the President.” ”I did it last week.”
注：如果上文表示否定意义而在简短反应中表示相同的看法时，则用“neither / nor +do +主语”。此时，do 不是动词替代词，而是助动词。例如：
Mary didn’t like English, neither did her brother.

3. to代替整个动词不定式
当某个动词不定式词组在句子中再次出现时，我们可以只用to这个词，而不必重复整个动词不定式词组。
例如：
I went there because I wanted to. (=…because I wanted to go there.)
Perhaps I’ll go to Brazil this summer; I’d very much like to.
“Are you and Gillian getting married?” ”We hope to.”
I think he should get a job, but you can’t force him to if he’s not ready (to).
I don’t dance much now, but I used to a lot.
We’ll never leave home; he hasn’t got the courage (to).
注1：有时to可省略（尤其是在形容词和名词之后，见例4、6）。在某些动词（如try）之后也可省略to。例如：
 “Can you start the car?” “Ok, I’ll try.”
注2：在want和would like之后，通常不能省略to。然而，当want或like 用于从句时（例如，在when, if, what, as之后），to常常省略。例如：
Come when you want.
I’ve decided to do what I like.
Come and stay as long as you like.