When to use era or fue

Question

Hi, could anyone give me a basic rule on when to use fue or era in the preterite'

Answer 1

The same as with any other verb in both tenses:

Imperfect is used for things not finished in the moment in the past we are considering (maybe they finish later, while still in the past, but that is no important when using this tense). Therefore, it is used, for example, to describe things in the past, or for things that used to happen regularly.

Preterite is used for things that ended (or started with verbs like "empezar") in the moment in past we are considering.

Answer 2

Did you mean to say "when to use the imperfect (era) or preterite (fue) form of ser to form the past tense'