accadesse

happened

→ accadere
accad to happen + esse imperfect subjunctive (io/tu/lui/lei)
verb · singular · imperfect subjunctive Rare #38,668
happened Imperfect subjunctive (used after expressions like “it was possible that…”, “it was necessary that…”, “I hoped that…”).
IT: Speravo che non accadesse nulla.
EN: I hoped that nothing happened.
Conjugation of accadere
Subjunctive imperfect
accadesse

Come ha potuto permettere che accadesse questo?

How could he let this happen?

Come avete potuto permettere che accadesse questo?

How could you let this happen?

Ho lasciato che accadesse.

I let it happen.

— L'ultimo imperatore (1987)

Come hai potuto permettere che questo accadesse?

How could you let this happen?