O Past Perfect é formado pelo verbo auxiliar "have" no passado (had) e o Past Participle do verbo principal.
Ex.: Thomas had been terribly disappointed when he met her.
When I arrived there, I saw that I had gone to the wrong place.
They had lived in the same building for two years and didn't know each other.
Had you been there before?
Had I worked with them?
I hadn't seen them until then.
Hugh hadn't realized that the situation was so serious.
Em um contexto já situado no passado, usamos o Past Perfect para mostrar que a ação à qual nos referimos é anterior às que acontecem no texto.
Ex.: I was talking to her, and suddenly, I realised that we had met once before.
Note que o Past Perfect deve estar sempre inserido em um contexto já no passado, ou sua utilização estará errada.
Fonte: www.brasilescola.com