muy facil, el tiempo es la diferencia. "era" es pasado imperfecto, "fue" es pasado perfecto, es decir, si escribes "era" quieres decir que algo tuvo una duracion considerable en el pasado, si tu escribes "fue" quieres decir que solo fue "una sola vez" y ya no volvio a ocurrir.
too easy, the time is the difference. "era" is imperfect past, "fue" is perfect past, ie, if you write "era" you mean that something have a considerable duration in the past, if you write "fue" you mean only happened "once" and it didn't happen again.
for example:
el juego era muy divertido (the game was very funny): this means usually the game was funny
el juego fue muy divertido (the game was very funny): this means always the game was bored but only once was funny.
tenia un juego, ERA divertido jugarlo solo pero cuando lo jugué con mis amigos FUE** la primera vez que lo disfrute realmente
I had a game, it was funny play it alone but when I played it with my friends it was the first time I really enjoy it.
**note: usually you can write "era" when you write "fue" but in this case is not correct to write "era" for the sentence.
I hope you understand, any question tell me please :)