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
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).