The imperfect subjunctive tense is formed by dropping the -ron ending from the third person plural preterite to form the stem for all regular and irregular verbs:
 

Although there are two forms of the imperfect subjunctive, the first form tends to be the most commonly used in spoken Spanish.  The second form, although used in Peninsular Spanish speech, is generally a more literary form.  For example, one would probably hear Mi padre no quería que yo me casara con Miguel more frequently than Mi padre no quería que yo me casase con Miguel (My father didn't want me to marry Miguel).

The imperfect subjunctive, like the present subjunctive, appears in subordinate clauses.  The imperfect subjunctive is most commonly used when the verb in the main clause requires a subjunctive in the past:  No queríamos que el gobierno pasara la ley (We didn't want the government to pass the law).  The imperfect subjunctive is also used when the verb in the main clause that requires a subjunctive is in the present indicative, but the subordinate clause expresses an idea in the past:  Es impresionante que él estudiara tanto (It's impressive that he studied so much).