Here is an excerpt from some Spanish literature:
A pesar de que existen muchas versiones de los origenes del mole poblano, la mas común dice que *fue inventado* en el siglo XVII.
Firstly why is the text highlighted in bold using the past progressive from.
Secondly, why is SER being used in place of ESTAR.
Thirdly, given that this is a completed event that only occurred once
why is the preterite form not used.
Thxs in Adv
Nev
Why is this tense used in this case?

nevjohnson
June 20, 2006

Lenguas
June 20, 2006
_fue inventado_ means _was invented_.
This _is_ the preterite form. The thing is that it is in the passive.
Here:
He invented - él inventó - Preterite active
He was invented - él fue inventado - Preterite passive
Past progressive:
He was inventing - él estaba inventando
He was being invented - I think such a thing isn't even used in Spanish
I hope that's clear now.