Can we use a gerund instead

Question

This is an excellent lesson. Very clearly explained.

I understand the idea of using the imperfect to picture what is happening in the moment. But there is another tense for this as well. I forget the name of it, so I will just use an example.

"La profesora abría la puerta."

"La profesora estaba abriendo la puerta."

Would the choice between these two tenses be up to the speaker, or would there be a grammatical rule that tells us which one to use?

Thanks.

Accepted Answer

Hola Marcos
The other option to the imperfect is the "imperfect progressive"(estaba trabajando, estabas trabajando, estábamos trabajando..) The progressive tense is always going to emphasise more the "progress" of the action, so if that is the speakers focus they'd choose it against the simple imperfect tense. 
That is the main function of the progressive tense while the simple imperfect may be used with other considerations like stating background information (not necessarily emphasising the progress so much). 
In real practice I am confident to say that there is a tendency to use the progressive form over the simple imperfect. The imperfect may also sound slightly formal while the progressive imperfect is more relaxed and informal. 
I hope this clarified it.
Sorry about the delay in answering.
Saludos