Past Perfect Tense

The Past Perfect expresses the idea that something occurred before another action in the past. It can also show that something happened before a specific time in the past. This is, the past of the past.

Examples:

The train left at 9am. We arrived at 9:15am. When we arrived, the train had left.

I had never seen such a beautiful beach before I went to Hawaii.
I did not have any money because I had lost my wallet.
Tony knew Istanbul so well because he had visited the city several times.

note: sentences in blue are in Simple Past tense

Past Perfect Structure:

[subject +had + past participle]

Examples:
You had studied English before you moved to New York. (Affirmative sentence)
Had you studied English before you moved to New York? (Interrogative sentence)
You had not studied English before you moved to New York. (Negative sentence)

	subject	auxiliary verb		main verb
+	I	had		finished	my work.
+	You	had		stopped	before me.
-	She	had	not	gone	to school.
-	We	had	not	left.
?	Had	you		arrived?
?	Had	they		eaten	dinner?

Contraction of Past Perfect:
When we use the Past Perfect in speaking, we often contract the subject and the auxiliary verb. We also sometimes do this in informal writing:

I had	→	I'd
you had		you'd
he had she had it had		he'd she'd it'd
we had		we'd
they had		they'd

Examples:

They'd gone home.

I'd eaten already.

In negative sentences, we may contract the auxiliary verb and "not":
I hadn't finished my meal.
Anthony hadn't had a day off for months.

taken from:

https://www.englishclub.com/grammar/verb-tenses_past-perfect.htm