Back by popular demand, more subjunctive for you.  This time we will look at the basics of the Past Subjunctive, also referred to as the Imperfect Subjunctive.  Please note the subjunctive tenses are advanced Spanish.  If you are relatively new to studying Spanish,  don't be discouraged if you find this difficult.  It is just another step in the learning of Spanish that you will come to at the right time.  For you advanced students that time has arrived!     I remember thinking,  yet another subjunctive tense?  After all,  if something has already happened,  then how can it be subjunctive?  Now I have to learn another one. Great.  But once I got into it a little, it was not that bad.  At that point I did have a fair understanding of the present subjunctive.     Just like the present subjunctive, the difficult part of the past subjunctive in knowing when to use it.  But before looking at what triggers a past subjunctive sentence, lets first look at  how the verbs in the past subjunctive are conjugated.  Lucky for us, this is not too difficult because all Spanish verbs in the past subjunctive are conjugated in the same manner.   Let me mention that there are two ways Spanish verbs are conjugated in the past subjunctive.  The one below is by far the most common.  Don't worry about the other.

The first step is to identify the verb in the simple past (preterite), they, (ellos) form.  This is also referred as the preterite, third person, plural.  Here are a few examples:
Hablar- hablaron.  They spoke.
Trabajar- Trabajaron. They worked.
Saber- Supieron. They knew.
Comer- Comieron. They ate.
Hacer- Hicieron. The made or did something.
Next, drop the 'ron' ending and replace it with 'ra' 'ras' 'ran' or 'ramos',  depending on what form is needed.  Take a look at how hablar, to speak, is conjugated in the past subjunctive:
Hablara,  yo, usted, ella, él, form.  I, you, she, he, it, spoke.
Hablaras, form. You spoke, informal. 
Hablaran, ellos form.  They spoke.
Hablaramos, nosotros form. We spoke.
All other Spanish verbs are conjugated in the same way.  All you do is identify the simple past, ellos form of the verb, drop the 'ron' and replace with the desired past subjunctive form ending.   
Now let's get right to it and see what causes a past subjunctive sentence.   Some of the  causes or triggers are similar for both the present and past subjunctive.  So,  if you have good understanding of when the present subjunctive is used,  it will help you understand when the past subjunctive is used.  
If you are a little rusty on the present subjunctive you may want to review the articles "Spanish Hopes, Desires, and Uncertainties" and "Perfect Spanish Uncertainties".   Also, you may want to review "A Not So Perfect Tense" as the imperfect tense is commonly used with the past subjunctive.  
Let's review several of the more common ways a sentence will be past subjunctive by comparing them to a similar sentence in the present subjunctive.  Subjunctive sentences will always have two parts and usually include two subjects.  The first part will most often require the second to include a subjunctive verb.  
Take a look at the differences between present and past subjunctive sentences:
A hope or desire, present subjunctive:
I hope that you will clean the house today. - Espero que limpie(s) la casa hoy.
I want them to clean the house. - Quiero que limpien la casa.
They insist that you learn Spanish. - Insisten que aprenda(s) español
These indicate a desire now for something that may or may not be done.  
Past subjunctive:
I hoped (I was hoping) that you cleaned the house. - Esperaba que limpiara(s) la casa.
I wanted them to sell the car. - Quería que vendieran el carro
They insisted that you learn Spanish. - Insistían que aprendiera español
These state what a desire was in the past that may or may not have happened.  They all start out  in the imperfect tense.  But a sentence does not necessarily have to start out in the imperfect.  There are a few examples of this below.
Doubt or ignorance, present subjunctive:
I doubt that they will arrive on time. - Dudo que lleguen a tiempo.
I don't think I know her. - No creo que la conozca
He is not sure that his car is ready. - No está seguro de que su carro esté listo.
Doubt or ignorance, past subjunctive:
I doubted that they would arrive on time.  - Dudaba que llegaran a tiempo.
I did not think that I knew her. - No creí que la conociera.  This starts out in simple past.
He was not sure that his car was ready. - No estaba seguro de que su carro estuviera listo.
 'As if ',  or 'even though' sentences, present subjunctive:
Even though he does not speak Spanish, he can learn. - Aunque no hable  español, puede aprender
Even if they are late, we will still have a good time. - Aunque llegen tarde, pasarémos buen tiempo todivía.  Notice the use of the future tense (pasarémos).
'As if ' sentences  past subjunctive:
Even though he did not speak Spanish, he could have learned. - Aunque no hablaba español, pudiera haber aprendido.  Notice the use of the imperfect tense (hablaba). 
It was as if she knew everyone. - Fue como si (ella) conociera a todos.  
Here are a few more examples of when a past subjunctive sentence can start out in the present, but refers to a past subjunctive condition.  Look at these examples:
I hope that my wife did not spend a lot of money!  ¡Espero que esposa no gastara mucho dinero!
I don't suppose you knew the answer. - No supongo que supiera la respuesta
They don't think they told the truth. - No creen que dijeran la verdad.
There certainly are other conditions a past subjunctive verb can be called for.  But if you understand a few of the basics, like the examples above, that will take you along way.  
Again, do not be discouraged if you find this difficult.  But at some point do try to become as familiar as you can with both the past, as well as the present subjunctive.  Spanish places a far greater emphasis on the subjunctive than does English.  And subjunctive sentences in Spanish occur all the time.
Practice basic subjunctive sentences with your Spanish teacher if you have one.  And always look for opportunity to use the Spanish you have learned in live conversation.   
If this introduction to the past subjunctive has helped you,  and I hope it has,  you are progressing far beyond  beginning Spanish.
Stay with it.