Substitution using so and do

The word so can be used as a substitute for an entire previous sentence:

Q. Will we have time for breakfast at the airport?

A. I hope so.
          (= I hope we will have time for breakfast at the airport.)

Using so in this way means that we can avoid unwieldy repetition.
The negative counterpart of so is not:

Q. Is Jim coming tonight?

A. I hope not.
(= I hope Jim is not coming tonight.)

So can also substitute for a phrase:

    The meat was very fresh and so were the vegetables.

Here, so substitutes for the adjective phrase very fresh. The negative counterpart
of phrasal so is neither:

   The meat was not very fresh and neither were the vegetables.

The verb do can also be used as a substitute:

      They asked me to drive them to the airport and I did.

Do sometimes combines with so as a substitute:

You should save a little money every month. If you do so, you will have no worries.

Here, do so substitutes for save a little money every month.