No fui inteligente aquella tarde y me equivoqué. The period of time I'm referring to is short. No era inteligente en mi juventud y me equivocaba a menudo. The period of time is longer and the actions happened repeatedly. Yo comí demasiado en la boda de mi hermano. Yo comía demasiado en aquella época. Yo leí un libro apasionante aquel año. Leía sin parar, a todas horas.

Hey Darky, No fui muy inteligente = At that moment, I was not very smart No era muy inteligente = In the past, I was not very smart Cheers!

They are both correct. The first is simple tense ,one action. The other is imperfect tense, action is not simple, no beginning or end. You can not say seramos, you must say eramos when using this verb. It changes that is why it is imperfect. Hope this helps.