It is difficult to explain. "Era" is imperfect tense so it refers to and action that occurs in numerous time and dont have a definite end, "fue" is preterite and refers to action in a definite time. I think is easier to explain with another verb, for example run. If I say "yo corria" (imperfect) it means that I ran for a period of time in the past but if I say "yo corrí" (preterite) means that I ran one time. Here in Spain many people use it wrong and dont know the difference so even if you use it wrong it is understandable

Era = Imperfect of "ser" Fue = Preterite of "ser" The imperfect is used to describe events without definite beginnings or ends, such as describing the circumstances in which an event took place and continuous actions in the past. The preterite, on the other hand, conveys actions that were started and finished in the past with definite beginnings and ends. For "ser", here are some examples. Ella era una niña cuando yo vivía en Valladolid. She was a child when I lived in Valladolid. The state of being a child is continuous in the past, and doesn't really have defined beginnings or ends. Similarly, you say "vivía" as opposed to "viví", because you lived in Valladolid for an extended period of time, without a specific timeframe. Here's an example with "ser" in the preterite. Él fue el padrino de bodas por su amigo. He was the best man for his friend. Here, the "él" in question was a best man, but his being the best man was a one-time thing, a condition that doesn't continue further in the past. I hope that helped!