When we want to say that something happened before some moment in the past, we use the verb had and we put –ed at the end of a verb. This grammar is called the past perfect.
I had arrived before 6.
The house was dirty. Nobody had cleaned it for years.
There are some verbs which have their special form for the past perfect. There are 47 of these verbs for Level 3. These verbs are
been, broken, built, chosen, come, cost, cut, done, drawn, drunk, driven, eaten, fallen, forgotten, found, frozen, got, given, gone, grown, had, heard, hidden, hit, kept, known, left, learnt, lost, made, met, paid, put, read, run, said, seen, sold, sent, slept, taken, thought, told, understood, woken up, won, written.
We had left before they came.
I thought that she had found my letter.
We use the past perfect in books when we tell a story.
If he had had such power, he would have been able to watch the sunset two hundred times on the same day.
You can see more examples in the book The Little Prince.