... tense is used to talk about the background information for a story

Often the past continuous action is interrupted by another (in the ...)

_____ I was doing my homework, my mother was sitting in the chair and watching a romantic comedy.

... is used to make it clear that one action happened before the other

"We couldn't open the door because Jackie had forgotten her keys at home".

What happened first?

When I got to the meeting hall I discovered I ... (not/take) my documents.

"Mark had lived in Vilnius for 2 years before he moved to Minsk."

Where did he live first?

I was listening to the radio when the accident ... (happen).

As soon as Jackie arrived home she realised she ... (forget) to buy products.

We often use the ... with 'thinking' verbs like remember, realise, think and etc.