Using the Past Perfect

We use the past perfect tense fairly often in English.

1: A completed action before something else in the past.

• When we arrived, the film had started. (= first the film started, then later we arrived)

2: To explain or give a reason for something in the past.

• I'd eaten dinner so I wasn't hungry.
• It had snowed in the night, so the bus didn't arrive.

3: Stative verbs only: something that started in the past and continued up to another action in the past.

• When he graduated, he had been in London for six years.
  
  (= he arrived in London six years before he graduated and lived there continuously until he graduated, or even longer)

4: As part of the third conditional.

• If I had known you were ill, I would have visited you.

• Try an exercise about the past perfect and past simple here
• Try an exercise about the past perfect and past perfect continuous here