Apuntes sobre el pluscuamperfecto/pluscuamperfecto del subjuntivo

El Pluscuamperfecto (The Pluperfect/Past perfect) - used to describe what had happened
before another past event or point of time in the past.
It is a compound tense that is formed as follows:
The imperfect of haber
había habíamos
habías habíais
había habían
Examples:
+
past participle
(Irregular past participles)
ar = ado
er/ir = ido
hacer = hecho
ver = visto
romper = roto
volver = vuelto
leer = leído
decir = dicho
poner = puesto
escribir - escrito
morir = muerto
oír = oído
This summer we went to Spain. We had never gone abroad before.
Este verano fuimos a España. Nunca antes habíamos ido al extranjero.
Yesterday I picked up the tickets that I had reserved last week.
Ayer recogí los pasajes que había reservado la semana pasada.
When Carlos arrived at the station, the train had already left.
Cuando Carlos llegó a la estación, el tren ya había salido.
El Pluscuamperfecto del subjuntivo (The Pluperfect/Past perfect subjunctive) –
used when showing emotion, hope, doubt (wedding trigger) in the past – about a past action that
happened BEFORE another past action or point in time. *(see above explanation of pluperfect) 
OR if you wish something had happened differently.
It is a compound tense that is formed as follows:
The imperfect subjunctive of haber
hubiera hubiéramos
hubieras hubierais
hubiera hubieran
+
past participle
*(Irregular past participles)
ar = ado
er/ir = ido
Examples: We were surprised that they had never gone to Spain before last summer.
Estábamos sorprendidos de que ellos no hubieran ido a España antes el verano pasado.
My friends were happy that I had picked up the tickets before meeting them.
Mis amigos estaban contentos de que yo hubiera recogido los billetes antes de reunirnos.
Carlos arrived at 2:00. He hoped that the train had not already left.
Carlos llegó a las 2:00. Él esperaba que el tren no ya hubiera salido.