Use pretérito imperfecto or pretérito indefinido?

Question

I have the following sentence :

Los invitados (llamar) ______ a la puerta y les (abrir) _____ una joven (ser) _____ una muchacha.

I think that it should be the following :

Los invitados (llamar) llamaban a la puerta y les (abrir) abrio una joven (ser) era una muchacha.

Yet, I'm not sure since (abrir) aka opening the door, is an action that terminated in the past she open the door the action was finished when the door was open if I use abria doesn't it mean that the action is still taking place? like she is just opening the door? I'm a bit confused.

Any ideas?

Answer 1

If you say

Los invitados llamaban a la puerta y les abrió una joven, era una muchacha.

it sounds like the door was opened while the guests were knocking at the door. You get the same meaning with

Los invitados llamaban a la puerta cuando les abrió una joven.
Los invitados estaban llamando a la puerta cuando les abrió una joven.

If the door was opened after the knocking, you must use llamaron:

Los invitados llamaron a la puerta y les abrió una joven. Era una muchacha.

Note that you must you a comma or period before the era. About your question, yes, you must use abrió as the action was executed once and has already finished. If you use abría it sounds as an action that was executed regularly, as in the following example:

Siempre que llamaban a la puerta les abría la misma muchacha.

Answer 2

Abrió is the correct one, since the action has already happened and finished.

Also the you have to use a coma after era:

Los invitados (llamar) llamaban a la puerta y les (abrir) abrió una joven (ser), era una muchacha.