La idea
Usamos el pasado perfecto para mirar hacia atrás desde un momento del pasado. Sirve para dejar claro cuál de dos acciones pasadas ocurrió primero.
Forma
had + past participle
Se usa la misma forma con todos los sujetos.
Afirmativa: They had watched three episodes.
Negativa: They hadn’t watched the final episode.
Pregunta: Had they watched the trailer?
Dos acciones en el pasado
El pasado perfecto normalmente describe la acción anterior. La acción posterior suele ir en pasado simple.
The episode had already started when we turned on the television.
We chose a different series because we had seen the first one before.
By the time our friends arrived, we had watched three episodes.
Palabras como already, just, before, after y by the time suelen ayudar a aclarar el orden de los hechos.