Yo estaba durmiendo cuando me llamaste.
Yo dormía cuando me llamaste.
Both have the same meaning of "I was sleeping when you called me". I can't really understand if there's any difference between imperfect indicative and imperfect progressive.
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In this sentence, and many of the kind, there is no difference between the imperfect and the progressive. I would personally use the progressive, but the form with the imperfect is grammatically correct and its meaning is clear.
The imperfect can also express a habitual action in the past (which in English would employ used to or the modal would), and in that usage you cannot substitute the progressive:
Yo jugaba mucho al fútbol cuando era pequeño.
"I used to play football a lot when I was little."
A habitual action in the past can employ a progressive form, but only in a construction using an adverb or phrase like siempre or todo el tiempo (this is mostly as in English):
Yo estaba siempre jugando al fútbol cuando era pequeño.
"I was always playing football when I was little."
This emphasizes the repetition or regularity of the action.
Some times the verb does not lend itself to use in the progressive, even in constructions exactly parallel to that of your example:
Tenía las manos ocupadas cuando tocaste a la puerta.
"I had my hands full when you knocked on the door."
(i.e. you cannot use *estaba teniendo las manos ocupadas).
In general I would advise that, for a structure like your example ("to be doing something (continuous) when something else (suddenly) happened"), you should use the progressive. The sentence will then have a main clause in the progressive tense and a temporal subordinate in the indefinite preterite.
Here's a paradigm I find helpful:
Estaba caminando a la casa cuando vino el aguacero. | I was walking home when the downpour started.
First, we set the scene; then comes something sudden, like a thunderclap.
Now consider this sentence:
Yo me preguntaba si me iban a invitar. | I was wondering if they were going to invite me.
Here I prefer "preguntaba" because "estaba preguntando" isn't really necessary, given that there's no sudden action. You could use the progressive here, but you don't need to, and the imperfect indicative is more elegant.
The presence of an adverb will often nudge one toward the imperfect indicative, for example:
Apenas empezaba a llover cuando llegamos. | It was just starting to rain when we arrived.
Without the adverb you could go either way (empezaba OR estaba empezando).
Note that in Mexico (at least), there is another way to construct the progressive: replace "estaba" with "iba" or "venía." Examples:
Suppose my son is telling me a story, and the order of the incidents has gotten a bit confused. I might ask
Espera, no entendí. ¿Venías caminando a la casa cuando pasó eso? | Were you walking home when that happened?
My partner asks me if I'm almost ready to leave the house. I respond
Iba buscando una carta que me llegó ayer pero mejor busco más tarde. Vámonos.
Iba caminando a la casa cuando vino el aguacero.
These three sentences feel very progressive.