fue vs era
Both are past tense of ser — fue is a completed snapshot, era is the background scenery.
fue
Preterite — the event happened, it's done, you're delivering a verdict on it.
Fue una noche increíble.
It was an incredible night.
The night is over. You're wrapping it up. Preterite = closing the bracket.
Fue mi mejor amigo.
He was my best friend.
That friendship has ended — he moved away, you fell out, or he died. Fue signals it's finished.
¿Cómo fue la entrevista?
How did the interview go?
Asking for an overall verdict on a completed event.
era
Imperfect — ongoing background state, what was true across a whole period.
Cuando era niño, era muy tímido.
When I was a child, I was very shy.
No specific moment — a quality that lasted across years of childhood.
Era un día tranquilo cuando llegó la noticia.
It was a quiet day when the news arrived.
Era sets the atmosphere; llegó (preterite) is the event that cut through it.
Era mi amigo.
He was my friend (back then).
Just remembering a time — no implication the friendship ended badly. Compare fue mi amigo.
Tricky bit
'Fue' vs 'era' with the same sentence changes meaning: 'fue mi amigo' (friendship ended) vs 'era mi amigo' (fond memory of a time). Native speakers feel this immediately.