The imperfect subjunctive or imparfait du subjonctif in french, is a simple tense. This tense is not formed with an auxiliary.
The imperfect subjunctive is always constructed from the past historic; hence, if the past historic does not exist (defective verbs) neither will the imperfect subjunctive. To be more exact, the imperfect subjunctive stem consists of the second person singular of the past historic, except that in the third person singular of the imperfect subjunctive the final -s- of the stem is replaced with a circumflex over the preceding vowel. The stem is otherwise stable for a single verb, and the endings are always: -se, -ses, -ˆt, -sions, -siez, -sent pronounced /s, s, -, sjɔ̃, sje, s/.
(Il fallait) que je chantasse, que tu finisses, qu'il bût, que nous vinssions, que vous parlassiez, qu'elles rougissent...
Imperfect subjunctive endings
The imperfect subjunctive endings are always for all three groups: -se, -ses, -ˆt, -sions, -siez, -sent pronounced /s, s, -, sjɔ̃, sje, s/.
Here are the complete endings in imperfect subjunctive for the three groups with affixe -ass- for the first group, affixe -iss- for the second group and affixes -iss-, -uss-, -inss- for the third group :