The past resultative (acababa de)

To express that someone had just done something in the past, use the imperfect tense of acabar de followed by an infinitive.

I had just answered the call.
Acababa de contestar la llamada.

This structure is perfect for talking about bad timing or sudden interruptions—like trying to make a phone call with a terrible signal.

I had just answered, but I had to hang up because of the awful Wi-Fi.
Acababa de contestar, pero tuve que colgar por el wifi pésimo.

Conjugating "acabar de" in the past

To form this, conjugate acabar in the imperfect tense, add de, and leave the action verb in its infinitive form.

SubjectImperfect of "acabar"
yoacababa
acababas
él / ella / ustedacababa
nosotros / nosotrasacabábamos
ellos / ellas / ustedesacababan

Notice how this structure sets up a background action that is often immediately interrupted by another past event.

He had just dialed when you decided to hang up.
Él acababa de marcar cuando decidiste colgar.
Since the signal was awful, we had just lost coverage.
Como la señal era pésima, acabábamos de perder la cobertura.
The Wi-Fi had just failed during the call.
El wifi acababa de fallar durante la llamada.