Why is "I had just" + infinitive translated to "acababa de" + infinitive, and not "acabé de"?

Question

In this question How is the imperfect of 'acabar de [+ infinitive] translated to English? there is discussion of the correct translation of acababa de + infinitive. To use the example given by @Gorpik in his/her answer

Yo acababa de cenar cuando llamaron a la puerta

which is

I had just finished my dinner when the door bell rang.

But why is acabar in the imperfect? We have two actions here: dining and the door bell ringing. When the second one happens the first one had already finished so it seem strange that we use the tense which normally refers to continuing, habitual or incomplete actions in the past for the first action. Having said that is does sound quite natural so it must be what I have heard in my limited experience so I imagine I have misunderstood the distinction between the imperfect and the preterite (as I would call them in English).

Answer 1

It is in the imperfect because it is not being used like acabar: "finishing" (1), but like acabar DE: "having just happened something" (11).

acabar

1. tr. Poner o dar fin a algo. U. t. c. prnl.
...
11. intr. Haber ocurrido poco antes algo. Acaba DE perder su caudal.

When you use it like that, you should be able to replace acabar de + infinitive with haber + participle (+ "brevity expression") without changing the tense. So:

• Acababa de cenar cuando llamaron -> Había cenado (poco antes) cuando llamaron -> I had just had dinner when they knocked
• Acaba de salir hacia el trabajo -> Ha salido (hace un momento) hacia el trabajo -> She just left for work
• Acabábamos de terminar de lavar el coche cuando empezó a llover -> Habíamos terminado de lavar el coche (segundos antes) cuando empezó a llover -> We had just finished washing the car when it started to rain.

Notice that, in the last example, "finish" corresponds to "terminar", not to "acabar". In your own example, you are translating "cenar" to "finish dinner" which would actually be terminar la cena. That might be the source of your confusion. You might want to use "have dinner" instead, like I did in the first item.

This use of acabar is NOT the same as estar acabando de, either, which would just be equivalent to "to be finishing".

If you understand enough Spanish, you may want to check the several relevant pages of RAE's Nueva Gramática, which fully explain this specific use of acabar with tons of examples and counter-examples.

Answer 2

Acababa is in the imperfect because, despite the fact that the translation is OK, it doesn't mean exactly the same as in Spanish. "I was finishing my dinner" would be more exact. That is, the action of finishing dinner was in progress when the door bell rang.

The Spanish equivalent of "I had just finished my dinner when the door bell rang" is straightforward: it uses the pluperfect.

Yo había acabado de cenar cuando llamaron a la puerta.

This is because there's already a simple preterit (llamaron) in the sentence, so referring to a previous action that has already been completed must be done using the compound tense. If not you could say simply:

Yo acabé de comer y entonces llamaron a la puerta.

Or with a different clause structure:

Apenas acabé de comer, llamaron a la puerta.

(Note this mirrors the English usage: "As soon as I finished my dinner, the door bell rang.")