We use the past perfect tenseA grammatical category that is marked by verb inflection. To express when an event or action happens in time or when a state exists, we use tenses. to express that an eventA general term expressing that somebody or something does something or that something happens. E.g.: An apple fell on Isaac’s head. took place before another event in the past, that is, it had been completeA concept, related to the perfect aspect, expressing that an action has finished. E.g.: He’s finished university.d. This tense allows us to keep track of the order of the events if this is not clear otherwise. Note that 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 something happened but rather that something happened before another past actionA general term expressing that somebody or something does something or a point of timeA particular moment in time. E.g.: She came home at six. ("six") I was born in 1980. ("1980") .

Julius Caesar attacked Britain in 54 BC. (This was a pretty long time ago, yet we use the past simple tense here.)
When I got home, they had already eaten everything in the house. (Maybe this happened only yesterday, still the point is that the eating happened before my arrival.)
They ate everything in the house. I got home too late.

It is important to remember that when you tell the events of a story in the order they occurred you do not need to use this tense.

ExpressionA word or group of words used in a particular situation. E.g.: Good morning, at six, on the sofas which are often used with the past perfect are by the time, no sooner, hardly, scarcely, barely, till, until, before.

By the time I got home, they had eaten everything in the house.
I had no sooner got home than the telephone rang.
The company had no sooner launched its new product than it went bankrupt.
I had hardly/barely/scarcely fallen asleep when there was an earthquake.
Little Edward had never seen a real cow until I showed him one last Friday.
He didn't leave until he had talked with the boss.
An internal investigation found no evidence that the data had been misused.

If we start the sentence with hardly, barely, scarcely or no sooner, we use inversionA structure in which a verb or an auxiliary precedes the subject. E.g.: "Here comes the sun.", that is, the word order changes:

No sooner had I got home than the telephone rang.
No sooner had the company launched its new product than it went bankrupt.
Hardly/Barely/Scarcely had I fallen asleep when there was an earthquake.

Diagrams

Form: past perfect
Past perfect Past perfect with superlative forms of adjectives