[<< wikibooks] Latin/Lesson 3-Subjunctive Imperfect
The conjugation of the subjunctive imperfect active follows a simple rule.  The verb in its infinitive form, that is, the second principle part, (amāre, to love, for example) simply has the active personal endings (-m/ō, -s, -t, -mus, -tis, -nt) added:
ego      amārem
tu       amārēs
is       amāret
nos      amārēmus
vos      amārētis
ei       amārent

the passive voice is formed by the addition of passive personal endings (-r, -ris (-re), -tur, -mur, -minī, -ntur) onto the infinitive stem

ego      amārer
tu       amārēris (-re)
is       amārētur
nos      amārēmur
vos      amārēmini
ei       amārentur

== Application ==
The imperfect subjunctive is only used for complex syntactic constructions; cum clauses and indirect questions and the like.  Rarely, if ever, does it stand alone.

=== Because/Cum ===
One application is in its use with 'cum' in the sense of 'because' as a clause. e.g. 

Diutius cum sustinere nostrorum impetus non possent, Helvetii se in montem receperunt.
When(Because) the Helvetis could not sustain/resist our attacks for long, they retreated to the mountains.