fosse stata

had been

→ essere
phrase Rare #43,800
had been Used for a feminine singular subject in the past subjunctive, often after expressions of doubt or in hypothetical situations.
IT: Pensavo che fosse stata una buona idea.
EN: I thought it had been a good idea.

Se ci fosse stata lì dentro May, mi sarei mangiato anche lei.

If May had been in there, I would have eaten her too.

— Hong Kong Express (1994)

Credeva che la scena in costume fosse stata fatta.

He thought that the costume scene had been filmed.

— Effetto notte (1973)

Se fosse stata un'aringa, l'avrei mangiata senza problemi.

If it had been a herring, I would have eaten it without any problem.

— Si alza il vento (2013)

In tutti questi anni ho pensato fosse stata la mia immaginazione.

All these years I thought it was my imagination.

— Arrietty - Il mondo segreto sotto il pavimento (2010)