El Pretérito Imerfecto vs. El Pretérito Indefinido in Gabriel Example

Question

Using El Pretérito Imerfecto in the Gabriel example above seems incorrect/confusing to me, also. Correct me if I'm wrong, but doesn't the imperfect here indicate a general inability to do something, like Gabriel just didn't know how to put the key in the keyhole (incidentally, the sentence should read "key in the keyhole", "keyS in the keyholeS", or "key in the keyholeS" if a single key fits two locks), which seems highly unlikely? It seems to me that El Pretérito Indefinido is more appropriate because: Gabriel didn't manage/succeed in putting the key in the keyhole, and
we are referring to a specific moment in the past and the time when it happened is relevant.

Answer

Hola Rand
The use of the Imperfect in this sentence that you are referring to:

Gabriel no podía meter las llaves en la cerradura anoche.
Gabriel couldn't put the keys in the keyhole last night.

is using this tense to simply mean that he couldn't/wasn't able to do it, but with no nuance at all in terms of "managing or not in the end".  If we use the Indefinido instead:

Gabriel no pudo meter las llaves en la cerradura anoche.
Carlos couldn't (=didn't manage/succeed) put the keys in the keyhole last night.

this implies that he couldn´t and the result was that he didn't manage to do it. We can see a negative result.The imperfect (no podía) doesn´t give any indication of the result (he couldn´t at then, but did he manage in the end? we don´t know.
I hope this clarified it.
In terms of using key or keys in singular or plural, well, I see what you mean, but you can also interpret our sentence as having a set of keys in his hand and not being able to fit any of them in one single keyhole (maybe he was a bit drunk?... :))
Saludos
Inma