[literacola]

Originally Posted by Rusty

estar + gerundio (estar escribiendo)
This structure is the progressive. This is used to describe action that is in progress (I was in the very process of writing, for example). Take care to not overuse this verb form; if you are not in the very process of writing something, choose the indicative mood instead.

estaba + gerundio (estaba escribiendo)
I was in the process of doing something when I was interrupted by something else. After the interruption, I returned to what I was doing.

estuve + gerundio (estuve escribiendo)
I was in the process of doing something when I was interrupted by something else. I did not return to that activity after the interruption.

imperfect indicative (escribía)
I was doing something. Indicates a setting of the stage for another event, a habitual action, or a repetitive action.

preterite indicative (escribí)
I did something. The action had a start and an end in the past.

I don't understand why estaba + gerundio would ever be used over the imperfect. Is it just that the subject returned to what they were doing?