Originally Posted by fernando el casir

The context is important. For example:
Yo era un niño cuando asesinaron a Kennedy. I was a boy when Kennedy was murdered.
Yo fui un tonto cuando vendí mi casa. I was a fool when I sold my house.

There are not interchangeable in those sentences. "Era" is an action that continued for certain time. "Fui" is something that only lasted a few moments. At least, that´s the way I use them.