French Imperfect Subjunctive
The French imperfect subjunctive is the imperfect tense of the subjunctive mood. It's a literary verb form reserved exclusively for formal writing, notably classic literature.
For example
Il fallait que vous fussiez prêts à 10h. - It was necessary for you to be ready at 10am.
Je doutais qu'il fût à la réunion. - I doubted that he'd be at the meeting.
See also: French Passé and All French tenses