Podía vs. pudo

Question

Gabriel no podía meter las llaves en la cerradura anoche. The fact that the sentence contains anoche, does this mean that is should be no pudoas it has a time of termination.

Answer

Hola Ralph
Not necessarily, no. Adverb "anoche" (last night) is one of those adverbs that would generally trigger the preterite as it indicates a time frame that is not connected to the present; the same goes for "el año pasado", "el otro día"... However, we can actually use these time phrases with the imperfect. When we use it with the imperfect, we "see" that action as an action happening "during that period of time", in this case "last night" but see the action as a non-completed action. It has more of a descriptive function. If we take the same sentence again and see both cases:
Gabriel no podía meter las llaves en la cerradura anoche.
Here I can just see in my head Gabriel trying to put the key in the lock. I am describing what he was doing at certain time during last night but this time is not relevant to me.
However:

Gabriel no pudo meter las llaves en la cerradura anoche.

Here what I see in my head is a very frustrated Gabriel after having tried hard to put the key in the lock but didn't succeed. I see this action as something finished. 
I hope this makes sense.
Saludos
Inma

Answer

This strikes me as an example of what Spanish speakers tell me often: if you use the imperfect, the listener is often times waiting for further information. If Gabriel couldn't get the keys in the door and then the building caught on fire, "podia" works in the context of a larger story. But in isolation, as these lesson examples must be, it seems to me that the preterite is really the best option.

Answer

Hola Allison
I see what you mean - the imperfect tense is very descriptive and it's used in narration to describe what was happening, describe the atmosphere, etc... so if you only use the imperfect when telling a story, the listener would be expecting at some poing some kind of action, otherwise it gets boring. 
Inma