Learn French: La pyramide a été construite par le roi.

In the passive voice, the past participle always agrees with the subject of the sentence. Since la pyramide is feminine, we add an e to the base participle construit to make it construite.

Why is it a été instead of est été, since the passive voice uses the verb être?

That is a great observation! While the passive voice does use the verb être, we have to put être into the passé composé here. The helping verb for être is actually avoir. So, the past tense of être is a été.

Since a été translates literally to has been, why does the English translation say was built?

In French, the passé composé is used for both the English present perfect (has been) and simple past (was) for completed actions. Since building a pyramid is a completed event in the past, translating it as was built sounds much more natural in English.

Can I use de instead of par to say by the king?

Not in this sentence. We use par to introduce the person or thing performing a specific physical action, like building. We only use de instead of par for verbs describing states of being, emotions, or conditions (for example, aimé de means loved by).