Forming the imperfect tense in French is easy. All that is required is the 'nous' form of the verb and an imperfect ending.
First we get the nous form of the regular verb and take off the -ons from it: Jouer -----> jouons -----> jou- Finir -----> finissons -----> finiss- Vendre -----> vendrons -----> vendr-
Next we add the appropriate imperfect ending: -ais -ais -ait -ions -iez -aient
There is only one irregular verb to this rule and that is étre. This verb rather than uses the nous form, uses ét-. The endings however are the same.