Ser y Estar, era vs. fue
¿Hay alguien que tenga información sobre cuando se usa 'era? o 'fue'?
por ejemplo, ¿es 'nadie sabe cuando fue creado el mundo? o 'nadie sabe cuando era creado el mundo'?
It's simple: you use imperfect ("era") if you want to recreate the moment from inside, as it happened, when you're considering its development, and not its start and end. You use the preterite ("fue") when you want to describe the moment from outside, and an specific end is needed, or implicit. If your intention is to talk about the creation of the world as it was happening, and not after its creation, then use the imperfect. If you want to look at the whole creation from an outside perspective, after it's done, then preterite is your choice. I believe the latter is the logical tense here.
You can use the same logic for most situations where a choice imperfect vs. preterite is needed.