Learn Spanish (Latin America): Fue al llegar cuando el guardia pudo acceder. - It was upon arriving when the guard was able to access.

This structure is a cleft sentence, which is used to emphasize a specific piece of information. By framing the sentence with Fue (It was) and cuando (when), we put a strong spotlight on al llegar. It highlights that the exact timing of his arrival was the key factor in getting access.

Why is the verb ser in the preterite (Fue) rather than the imperfect (Era)?

In cleft sentences, the verb ser generally matches the tense of the main action being described. Because the main verb pudo is in the preterite tense to show a completed action, we use the preterite Fue to keep the timeframe consistent.

What is the function of al llegar in this sentence?

The combination of al plus an infinitive translates to 'upon doing something' or 'when doing something'. Here, al llegar means 'upon arriving'. It is a very common and efficient way to express that one action happens immediately after another.

Why is pudo in the preterite instead of the imperfect (podía)?