1) Together with the Simple Past
Mary had read the book before she watched a film.
2) the past equivalent of the Present Perfect
He had played hockey.
no unambiguous ones
had + past participle
I had played hockey.I'd played hockey.
You had played hockey.You'd played hockey.
I had not played hockey.I'd not played hockey.I hadn't played hockey.
You had not played hockey.You'd not played hockey.You hadn't played hockey.
Had you played hockey?