Dear N:

You wrote:
> Suppose one is asked a question like:
> -What is Harry doing?
> with the present continuous tense.
>
> If things are happening very fast, could he use the simple present tense
> in his reply, as in:
> -He breaks the window, jumps out and runs behind the car.
>
He could, but he would give himself away as not being a native speaker. 🙂

If things are happening very fast, he would put the first two items in the past tense (since they have already happened) and the third in the progressive (or continuous) tense:

He broke the window, jumped out, and is running behind the car. Or more likely, he would put it into two clauses:
He broke the window and jumped out, and now he is running behind the car.

Unless the three things happen simultaneously, the tense helps to show the progression of the action.