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
