网友对这个问题的理解有些狭隘。不仅do有替代作用，所有的操作词都有替代作用的。因此，do起替代作用时，有时候是助动词do（擦作词），只能用于一般过去时和一般现在时。有时候do为替代动词（实意动词），这是do可以用非限定形式。

He doesn't like it, but I do.

I don't want it, but he does.

We didn't go there, but they did.

以上三句中的do,did都是助动词（操作词），只可以用一般现在时和一般过去时。其它操作词具有同样的作用。例如：

He isn't a student, but I am.

He can't do it, but I can.

He won't go, but I will.

当一个句子中没有操作词时，这个句子一定是一般过去时或一般现在时，这种情况下才用助动词do作为操作词，起操作词的替代作用。例如：

He bought the book, but I didn't.

He likes football, but I don't.

以下两个句子中都已经有了操作词，而操作词本身就有谓语替代作用，这时用操作词替代。

He wasn't doing his homework, but I was doing.

这句话为语法错误。正确的句子应该是：but I was.

He won't go there, but I will do.

这句话没错。如果不用do,则为操作词will替代。如果用will do，则为替代动词do的不定式替代了go there。

类似的do作为实意动词起替代作用的例句如下：

Lions can't climb trees, but leopards can do. do为实意动词，替代climb trees.

Tom writes more poems than he does short stories. does为实意动词，替代writes.