Acabar de is a periphrasis, in Spanish perífrasis verbal. A periphrasis is formed by combining two verbs. When this occurs, the first verb is conjugated, while the second verb remains in the infinitive form, many useful expressions are formed like this, that you have probably already seen:
Voy a comprar una casa (I am going to buy a house).
Tengo que hacer la maleta (I have to pack the suitcase).
1. One such expression combines the verb ACABAR (conjugated) with an infinitive. The preposition «de» is always used. The formula is: acabar de + infinitive (to have just done something, in the very recent past):
Acabo de llamar a mi madre (I have just phoned to my mum).
Acabamos de ver esa película (We have just seen that film).
Acababamos de salir cuando empezo a llover (We had just left when it started to rain).
2. Be careful, because this periphrasis in a negative sentence has a completely different meaning. No acabar de indicates that an action is not completed as expected or desired.
No acabo de entender lo que escribe (I just don’t understand what he writes).
No me acaban de convencer sus explicaciones (I’m not completely convinced by his explanations).
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