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.