The uses of the past subjunctive are mostly the same as those of the present subjunctive.  So the hard part- What the heck is the subjunctive? and, When the heck do you use it?- is over.  You merely have to learn a new tense of this mood.  Actually, there is one more use of the subjunctive which applies only to the past subjunctive- contrary to fact clauses (If I were......, [But I am not....]).  We'll deal with  those shortly.

Let's start with the formación del imperfecto (pasado) del subjuntivo.  Hay dos formas.  You need to know how to form one - the "RA" one- and you will also need to recognize the other- "SE"-  form.  Just as we did with the present subjunctive, we'll need to draw on previous knowledge in order to form the imperfect subjunctive.  As you remember, we used the yo form of the present indicative as the root for making the present subjunctive.  Why?  I have absolutely no idea.  Well we're going to use the third person plural (ellos) form of the preterite to form the imperfecto del subjuntivo.  Here's the good news:  There are no exceptions to this.  Every single verb uses the ellos form of the preterite as the root for the past subjunctive.  Here's the other news:  Remember how many rules and exceptions there were to forming the preterite?  So before we go any farther, review the preterite tense formation and pay close attention to the ellos form.  Take quizzes and send me the results;  it will be good for you to review the entire tense.
 

• Regular preterite tense formation

• Preterite of Ir, Ser, Dar & Hacer

• Preterite of stem-changing verbs

• Preterite of orthographic (spelling) changing verbs

• Preterite "jeron" verbs

• Preterite - The rest of the irregulars

"RA" form- the one I want you to use.

• Take the ellos form of the preterite- COMER>  "comieron"

• Drop the "ON" > "comier"

• Add these endings:  a, as, a, amos*, ais, an  *In nosotros you must add a written accent to the vowel before the "R".>comiera, comieras, comiera, comiéramos, comierais, comieran

• Every single verb follows this pattern.  No exceptions.  None.

• Take the ellos form of the preterite:  COMER> "comieron"

• Drop the "RON" > "comie"

• Add these endings:  se, ses, se, semos*, seis, sen *In nosotros, add a written accent to the vowel before the "S". > comiese, comieses, comiese, comiésemos, comiesen

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