The Imperfect Subjunctive is used in the same way as the Present Subjunctive only the action is in the past. You have to remember that this tense has nothing to do with the normal Imperfect Tense.  In fact in order to form this tense first we must go to the 3rd person plural (last person) of the Preterite Tense. E.g. Hablar – hablaron – take off the ron and add the following endings : ra,ras,ra,ramos,rais, ran.  There is only one pattern for AR, ER and IR verbs and there are no irregular verbs. 

EXAMPLES

Él quería que vinieran a la fiesta.  He wanted them to come to the party

Ellas creen que el era el ladrón. –  They think that he was the thief.

La madre espera que los niños se lavaran las manos.  The mum hopes that the boys washed their hands.