1. Действие, произошедшее раньше другого события в прошлом:

By the time the UN task force arrived, the rebels had taken the province.
At the time of her trial last year, HInkley had been in prison for eight months.

СРАВНИТЕ
When we got back, the babysitter went home (когда мы вернулись, (потом) она ушла)
When we got back, the babysitter had gone home (когда мы вернулись, она уже ушла)
НО
Если последовательность очевидна, Past Perfect не используется:
I opened the door and let him in (NOT I had opened the door and let him in)
I fell and broke my leg (NOT I had fallen and broke my leg)

2. Незавершённое действие («не пробыл я там и двух минут», «не прошёл он и ста ярдов», «не успел он войти в комнату»)

Positive form
He had scarcely entered the room when in a chair by the door he perceived Ann Chester.
She had hardly sat down when a stout boy flopped into the chair opposite hers.
No sooner, however, had they established themselves in the house than he perceived to his dismay the return of his gloomy mood.

Negative form
I hadn’t been in the pub two minutes when somebody brought Tom in for a drink.
They hadn’t gone four miles before he understood that it was going to rain.

He hadn’t been there for two days before he admitted that he shouldn’t have accepted the invitation.

Positive in form BUT negative in meaning
I realized before you had been here a fortnight that you never were cut for this life.
I discovered the news before I had been in the house for an hour.
I left university before I had taken the final exams.

3. В сообщении о прошедшем событии:

‘I’m sure I have met her before’ I told the policeman I had met her before (NOT … I met him before)
‘The village hasn’t changed much’                 I found that the village hadn’t changed much (NOT … the village didn’t change…)

4. It / this / that was the first time / the second time, etc.

That was the first time we had been to Scotland, so it was all new to us.
This was the second time Rachel had forgotten to give me a message.
It was the first time I had talked to her outside the office.
+
После прилагательных в превосходной степени (часто с “ever”)
That was the best film I had ever seen.
It was the most expensive hotel we had ever stayed in.

Just – подчёркивает, что действие произошло только что:
We wanted to talk to the landlady, but she had just left.

Already – подчёркивает завершённость (иногда с оттенком «даже раньше, чем ожидалось»)
When we looked out the window, we saw that the guests had already arrived.