The Present Perfect denotes a completed action connected with the present.
Stop that car! They have killed a child.
I am a little frightened for I have lost my way.
Tom is looking for his key. He can’t find it.
He has lost his key.
“He has lost his key” means that he lost it a short time ago and he still doesn’t have it.
The Past Perfect denotes an action completed before a certain moment in the past. The moment may be indicated by another past action expressed by a verb in the Past Indefinite or by an adverbial phrase.
They had walked only a few steps when a second group of tank drew up on the side road.
After she had cried out, she felt easier.
Fortunately the rain had stopped before we started.
I went to a party last week. Tom went to the party too. Tom went home at 10:30. So, when I arrived at 11:00 Tom wasn’t there.
When I arrived at the party, Tom wasn’t there.
He had gone home.
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