①過去完了形:had+~ed(過去分詞) の形をして、過去よりも前の 1.完了(結果) 2.継続 3.経験 を表す
1. Our train had already left(過去完了) the station when we arrived(過去) there.
2. My friend Anne had been(過去完了) in hospital for a week when I visited(過去) her.
3. I recognized(過去) Bob at once because we had seen(過去完了) each other several times.
②大過去:had+~ed(過去分詞)の形をしているが、ただ単に過去よりも前の時制を表すだけ
I lost(過去) my umbrella which I had bought(過去完了) about two weeks before.
参考にしてください。