The Past Perfect denotes an action completed by a definite moment in the past.
They had completed the construction by the end of April.
Они закончили строительство к концу апреля.
When director returned from Vienna, we had completed writing our program.
Когда директор вернулся из Вены, мы закончили писать нашу программу.
This definite moment can be expressed in two ways, - either by an adverbial phrase with the preposition by or by another action in the Past Indefinite.
The Past Perfect is used to transfer from a moment in the past to further past.
I couldn't remember the title of the article (which) he had recommended me to read.
Я не могу вспомнить название статью, которую он порекомендовал мне прочитать.
The Past Perfect can be used emphatically instead of the Past Indefinite in adverbial clauses of time when the speaker wants to emphasize that the action in the principal clause only took place after the action in the subordinate clause.
He agreed to continue the work after they had paid him part of money.
Он согласен продолжать работу только после того, как они заплатят ему часть денег.
The Past Perfect Inclusive denotes an action, which began before a definite moment in the past, had been going on for a certain period of time and at that moment was still going on. Normally, the Past Perfect Continuous is used here and the Past Perfect Inclusive replaces it when the use of the Past Perfect Continuous is impossible.
She said that she had known him for a few years.
The Past Perfect is used in the first part of sentences with the conjunctions no sooner ... than, hardly ... when, scarcely ... when; the world order in such sentences is often inverted - the auxiliary verb had stands before the subject.
No sooner had I taken off my coat, then the phone rang.
Как только я снял пальто, телефон зазвонил.
Hardly had we set down at the table, when Boris came.
Как только мы сели за стол, пришел Борис.
The Past Perfect can be used in combination with the Past Continuous when there are two actions, the first completed by a definite moment in the past and the second which was going on at that moment.
He looked through his field-glasses. The ships had left the port and were going North.
Он посмотрел в бинокль. Корабли покинули порт и двигались на север.
I came into the hall. The musicians had turned their instruments and were playing a sonata by Mozart.
Я вошел в зал. Музыканты настроили инструменты и играли сонату Моцарта.