The pluperfect subjunctive tense (sometimes called the past perfect subjunctive) is constructed by using the imperfect subjunctive form of the irregular verb haber with a past participle:
 

The past participle does not change forms when it is used in the pluperfect subjunctive tense.  For example, él hubiera leído, yo hubiese leído, vosotros hubierais leído, etc.  Although there are two forms of the pluperfect subjunctive, the first form tends to be the most commonly used in spoken Spanish.  The second form, although sometimes used in both Latin American and Peninsular Spanish speech, is generally a more literary form.  For example, one would probably hear Me dió mucha alegría que mi hermana hubiera ganadoel premio more frequently than Me dió mucha alegría que mi hermana hubiese ganadoel premio (I was very happy that my sister had won the award).

Like other subjunctive tenses, the pluperfect subjunctive is found in the subordinate clause of a sentence. Specifically, it appears in subordinate clauses in which the action of the verb has occurred prior to the action of the verb in the main clause:  Nadie pensaba que los pasajeros hubieran podido sobrevivir el accidente (No one thought that the passengers could have survived the accident).