Learn Spanish (Latin America): Como nadie contestó, acababa de dejar un mensaje en el buzón. - Since nobody answered, I had just left a message in the voicemail.

The phrase acabar de followed by an infinitive (like dejar) means 'to have just done' something. When you put acabar in the imperfect tense (acababa), it describes an ongoing state in the past, meaning someone had just done something right before another past event. So, acababa de dejar means 'I had just left'.

How do we know acababa means 'I had just left'? Couldn't it be someone else?

Yes, it absolutely could! The imperfect form acababa is the exact same for 'I', 'he', 'she', and 'you' (formal). Out of context, this sentence could just as easily mean 'he had just left a message.' The translation uses 'I' as one natural interpretation, but in real life, the surrounding conversation would tell you who made the call.

I learned that como means 'how' or 'like'. Why does it mean 'since' here?

When como is placed at the very beginning of a sentence to introduce a cause or reason, it translates to 'since' or 'because'. It sets up the background for the rest of the sentence: Since nobody answered, I did something else.

Why isn't there a no before contestó? I thought Spanish required double negatives.