The expressions do so, do it and do that are often used to avoid repeating a verb and its object or complement. These structures are rather formal.
- He asked me to get out and I did so without protesting.
- ‘Move those books off that chair.’ ‘I have already done so.’ (= I have already moved those books off that chair.)
- ‘Send them a fax.’ ‘I will do so.’ OR ‘I will do it.’
- I promised to buy her drinks and I did so. OR I promised to buy her drinks and I did it.
- I would like to ride a camel. I have never done that before.
- ‘I rode a camel during my trip to Morocco.’ ‘I would like to do that.’
Only do can be followed by so, it and that in this way. We do not use these words after other auxiliary verbs. For example we cannot say, I can so or You must it. But we can say: I can do so or You must do it.