Past perfect for actions completed before a point in the past

The past perfect tense is used to express that an action was completed before a point in time in the past. This point can be implied or expressed with a time expression or a clause with a verb in the past simple tense:

They had all arrived by 10 o'clock.
When he tried to stand up, he realised he had broken his ankle.

Contrary to a common misunderstanding, the past perfect does not express that something happened a very long time ago. What it expresses is not how long ago an event happened but rather that it happened earlier than something else:

Julius Caesar attacked Britain in 54 BC. (This was quite a long time ago; still, the past simple tense is used here.)
When I got home, they had already eaten everything in the house. (Maybe this only happened yesterday, but the point is that the eating happened before my arrival.)

Expressions which are often used with the past perfect tense: by, till, until, before, when, by the time, no sooner, hardly, scarcely, barely.

Danny had never seen a real cow till I showed him one last Friday.
He didn't leave until he had talked with the boss.
By the time she got to the theatre, the play had already begun.
I had no sooner got home than the telephone rang.
I had hardly/barely/scarcely fallen asleep when there was an earthquake.