do so/do it/do that

do 在这里不是助动词,而是实义动词(full verb),而且是及物的。so 是起替代作用的副词(有人把它看作是代词),it 是人称代词,that 是指示代词。应当注意,so, it 和 that 所代替的并不是前面某一个词,而是前面的谓语部分或者动词及其连带成分。

1. She hoped that he would search the room carefully, and he {searched the room carefully. did so. (CGEL) 她希望他仔细地检查房间，而他这样做了。

2. At night, when you really wish to go to sleep, you rarely manage to do so. (NCE, 3, L. 44) 夜里当你真想入睡的时候，你却很少能成眠。

3. We still think it a miracle that they were able to do so. (NCE, 3, L. 56) 他们竟然做到了这一点, 我们至今仍然认为是一个奇迹。

4. I could run after a bus and catch it twenty years ago, but I can't do that now. (CGEL) 20年前我能追赶公共汽车并把它追上，现在可不行了。

现在来回答你提出的第二个问题。do so, do it 和 do that 的意义和用法极其相近。对于初学者来说，只要知道它们常常可以互换使用，就足够了。可以不去深究它们之间的细微差别。例如：

1. A: Rover is scratching the door.

B: Yes, he always $\$left\{ \begin{array}{l} \text{does so} \\ \text{does it} \\ \text{does that} \end{array} \right\} when he wants attention. (CGEL)

A: 罗弗在抓门。

B: 是的, 当它想要人注意的时候, 总是这么干。

Mr Brown goes to the hospital for treatment every week: in fact, he has been $\$left\{\begin{array}{l} doing so \\ doing it \\ doing that \end{array}\right\} ever since I have known him. 布朗先生每周去医院治病:事实上,自从 我认识他以来,他一直就是这样。