Past Perfect Tense
timeline, form, uses

to show that an action was completed in the past before another action happened
She had read the book before she went to the see the film.
Robert had not studied English before he went to England.
(Notice the 2nd verb is in the Past Simple)

with indirect speech (also called reported speech)
She told the teacher that she had not done her homework.

with ‘if’ conditional clauses
If I had not washed my car, it probably would not have rained today.
If she had eaten her vegetables, her mother would have let her have pudding.

to express regret about something in the past
I wish I had listened at school. 
He wished he hadn’t spent all his money on the car.