French Subjunctive Pluperfect
Le Subjonctif Plus-que-Parfait is an extremely rare, extremely formal verb form. It's the literary equivalent of the French past subjunctive and is conjugated with the imperfect subjunctive of the auxiliary verb (avoir or être) + past participle.
Je ne croyais pas qu'il eût fini. - I didn't think he had finished.
Il était étonné que je fusse parti. - He was astonished that I'd left.