Is it really that simple?

Question

While I understand the explanation via timeframes and don't want to complicate things unnecessarily, this made me think:Can't I use the imperfect (progressive) with such timeframes, too?Ayer estaba limpiando la casa. (Should be ok.)Ayer entre la 1 y las 2 estaba limpiando la casa. (?)Ayer entre la 1 y las 2 estaba limpiando la casa cuando algo me asustó. (Should be ok.)That
 is, can't the imperfect (progressive) be used with an explicit 
timeframe, too, as long as its function is descriptive or establishing a
 background, i.e. as long as the aspectual semantics are imperfective 
within the timeframe?

Silvia Piriz · Accepted Answer

Hola D. L.
Yes, you’re absolutely right that the Imperfect Progressive (estaba + gerundio) can also be used with explicit timeframes. The key is the function of the tense. If the action is being presented as background or ongoing during that span of time, then estaba limpiando works perfectly.
For example: 
Ayer entre la 1 y las 2 estaba limpiando la casa
is fine if the speaker is simply describing what was happening in that period without emphasising whether it started or ended within that frame.
On the other hand, if the intention is to highlight that the action was contained and completed within those limits, then the Preterite Progressive (estuve + gerundio) is more natural.
In this case: 
Ayer entre la 1 y las 2 estuve limpiando la casa
presents the action as bounded, a task neatly enclosed within the timeframe.
As you mentioned, the Imperfect Progressive is especially useful when setting the background for another event.
For example: 
Ayer entre la 1 y las 2 estaba limpiando la casa cuando algo me asustó
clearly establishes an ongoing action that is interrupted.
So, yes, the Imperfect Progressive can appear with explicit timeframes, but the nuance is different from the Preterite Progressive. One describes an action as ongoing, while the other presents it as fully bounded within the limits. 
Hasta pronto
Silvia

D. L. · Answer

Thanks for the explanation, Silvia!