El Past Perfect Simple

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AFIRMATIVA:
Sujeto + had + participio pasado del verbo principal.

I had finished my homework by 9 pm

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NEGATIVA:
Sujeto + had not + participio pasado del verbo.

He had not finished his hemowork by 9 pm

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INTERROGATIVA:
Has + sujeto + participio pasado del verbo principal.

Had you finished your homework by 9 pm

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El Past Perfect Simple (Pretérito plucuanperfecto) se emplea para referirnos a una acción que ocurrió antes que otra también pasada y que suele ir en Past Simple.

When he had finished his homework, he went to bed (se traduce como Pretérito anterior: "hubo acabado")
We left the house because they had all gone to bed

EXPRESIONES TAMPORALES

Para unir la oración en Past Perfect Simple con la que va en Past Simple se usan conjunciones y expresiones como when, by the time, before y after, que ya indican qué acción ocurrió en primer lugar.

He had washed his hands before he ate his sandwich
He ate his sandwich after he had washed his hands

Además de las ya mencionadas, también podemos utilizar otras expresiones temporales, como just, already, until, never, etc.

They had just finished painting when Peter went to help them
He had already cooked the dinner when she arrived

Nota: Si dos acciones ocurren casi simultáneamente en el pasado, ambas van en Past Simple.

When he discovered the body, he began to cry

Es incorrecto decir: When he had discovered the body, he began to cry