DO SO 可以指代句子的一整个动词,包括它的宾主,修饰语:

  Quinn did not eat dinner quickly, but her brother DID SO.

 

在这里,do so 起的作用就相当于一个代词,而有时我们也省去so :

  Quinn DID NOT eat dinner quickly, but her brother DID.

 

DO IT 则就要明确指前面的名词:

  Quinn failed to do the homework, but her brother did IT.

It 指向了the homework,而这句话可以省略do

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