Past perfect tip: 

This verb tense is often used to express a cause and effect relationship in the past, made explicit by including the keyword because.

I went back home because I had forgotten my wallet.

My smartphone service stopped because I hadn’t paid the bill. 

Another past perfect tip: it’s very good to use past perfect with when in a sentence involving two actions as it clarifies the chronology. 

Compare the difference:

When I arrived at the club, the band had already started playing. 

When I arrived at the club, the band started playing.