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.