acabar de

A fixed idiomatic construction meaning 'to have just done something': 'acabo de comer' = 'I just ate'. In the present tense it describes the very recent past; in the imperfect (acababa de) it describes something that had just happened relative to a past moment.

• Acabo de llegar a casa. — I've just gotten home.
• Acababa de salir cuando empezó a llover. — I had just gone out when it started to rain.
• ¿Acabas de hablar con ella? — Did you just talk to her?

Don't use the preterite of acabar (acabé de) to mean 'I just did' — that reads as 'I finished doing' something, not 'I recently did' it; the present (acabo de) covers 'just did' relative to now, and the imperfect (acababa de) covers it relative to a past moment.