El Imperfecto de Subjuntivo

The past subjuntive is formed by dropping the "-ron" of the third person plural preterite and adding the following endings*:
 

ending 
yo -ra
-ras
él, ella, Ud.  -ra
nosotros/ as  -ramos
vosotros/ as  -rais
ellos, ellas, Uds.  -ran
* There is a second set of past subjunctive endings: -se, -ses, -se, -semos, -seis, -sen which are rarely used in our days.
 It is important to use the third person plural preterite, the "Uds. -form", because if it is irregular, the subjunctive forms are also irregular, and we know that many verbs are irregular in preterite. So, if you are not sure, use the Verb Conjugator to find the correct form. Some irregular verbs are:
 
andar anduviera
caer cayera
construir construyera
conducir condujera
dar diera
decir dijera
dormir durmiera
estar estuviera
haber hubiera
ir fuera
leer leyera
oir oyera
pedir pidiera
poder pudiera
poner pusiera
querer quisiera
saber supiera
seguir siguiera
ser fuera
tener tuviera
venir viniera