The past perfect simple is used to talk about events which happened in the past before another event also in the past. It is mainly used thus:
I had walked for about twenty minutes before I realized I’d left my wallet behind.
It’s usually followed by the past simple. In this way we use the past perfect simple to provide background the main event in the past. For example:
The King died. The Queen died three days later.
The Queen died three days after the King had died.
And here:
She had attended many interviews before she found the right job.
In everyday speech the past perfect simple is often dropped and the past simple used instead:
She attended many interviews before she found the right job.
Form
We make the past perfect simple by using had and the -ed form of the verb (the past participle):
{subject} + {had} + {past participle}
We did not all pass the exam though we had studied hard.
The Queen died of a broken heart after the King had left her.