Past Perfect

Время Past Perfect
Используется в 2 случаях:
1.Когда говорится о действии, которое произошло раньше другого, при этом более позднее действие выражается временем Past Simple.
Например: Когда я приехал в аэропорт, мой самолет уже улетел. - When I arrived at the airport, my plane had already flown away.
Она начала писать письмо после того, как ее дети ушли в школу.- She started writing the letter after her children had gone to school.
Перед тем как пойти в школу она сходила в магазин.- Before she went to school she had gone to the store.
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2.Когда говорится о действии, которое закончилось в прошлом к конкретному моменту времени в прошлом.
Например: Я закончила делать домашнее задание к 4 часам.-I had finished doing my homework by 4 o`clock.

Небольшой текст из учебника Changing times, хорошо иллюстрирующий использование Past Perfect:

In 1912, the Titanic hit (наткнулся) an iceberg on its first trip across the Atlantic, and it sank (затонул) four hours later. At that time, the Titanic was the largest ship that had ever traveled on the sea. It was carrying 2207 people, but it had taken on enough lifeboats (шлюпок) for only 1178 people. When the passengers tried to leave the ship, only 651 of them were able (смогли) to get into lifeboats (шлюпки).
The Carpathia was 58 miles away when the Titanic called on its radio for help. It arrived two hours after the great ship had gone down, and it saved 705 people. Some of the survivors (оставшихся в живых) had been in the icy water for hours when they were saved. Most of the passengers hadn't lived that long; 1502 people had lost their lives.
Through the whole tragedy, the Californian was only ten miles away. Its officers were close enough to see the Titanic, but they didn't understand the situation. They never received the Titanic's call for help, and they didn't come to the rescue until too late.
Why was there such a great loss of life? Why were there so few survivors (выживших)? Why didn't the Californian come to help?
First of all, nobody had prepared for such a tragedy. Nobody had believed that the Titanic could sink (затонуть). The steamship company (пароходная компания) had thought that its ship would be completely safe (полностью в безопасности) in all situations. They'd supplied lifeboats for only half the people. The passengers had not received their lifeboat numbers, nor had they practiced lifeboat drill (учения/тренировка) before the accident.
The ship had already received six ice warnings on its radio when it struck the iceberg. Nevertheless (тем не менее), it had not changed its direction or its speed. It was impossible to change direction quickly enough when the iceberg came in sight (в поле зрения). When the Titanic hit the iceberg, the radio officer on the Californian had just gone to bed. He'd tried to warn the officers on the Titanic about the ice before he'd gone to bed, but the officers hadn't listened.
After this accident, ocean travel changed. Now there are always enough lifeboats for everybody. Ships don't go so far north in winter, and they watch carefully for ice. Radio officers work 24 hours a day. A tragedy like the sinking of the Titanic should never happen again.

Отличное упражнение на отработку Past Perfect (с ключами):
http://www.englishpage.com/verbpage/verbs11.htm