From Gerald Erichsen,
Your Guide to Spanish Language.

Definition: A verb tense used to indicate that an action was completed at some specified or implied time in the past.
In English, the pluperfect is expressed by using the auxiliary verb "had" followed by the past participle: "She had studied."

In Spanish, the pluperfect is formed in the indicative mood by using the imperfect form of haber (había, habías, había, habíamos, habíais, habían) followed by the past participle: Había estudiado.

In Spanish, the pluperfect can also be used in the subjunctive mood by preceding the past participle with the imperfect subjunctive of haber (hubiera, hubieras, etc.): Dudo que hubiera estudiado (I doubt she had studied).

The pluperfect usages of the two languages are almost identical, so it is usually safe to translate the pluperfect of one language into the pluperfect of the other.

However, in some contexts, especially in colloquial usage, the simple past tense can be substituted for the pluperfect in either language.
Also Known As: "Past perfect" in English, pluscuamperfecto or pasado perfecto (less common) in Spanish.

Examples: Where had you gone? (¿Adónde habías ido?) I believed I had broken my leg. (Creía que se me había roto la pierna.) If John had had choices, surely he would not have gone to the United States in order to work. ( Si Juan hubiera tenido opciones, seguramente no se hubiera ido a Estados Unidos para trabajar.) It was not certain they had bought the book. (No era cierto que hubieran comprado el libro.