I had been working… (past perfect continuous)

POSITIVE:  I / They / We / You + HAD BEEN + working / eating 

He / She / It + HAD BEEN + working / eating 

NEGATIVE:  I / They / We / You + HAD NOT BEEN + working / eating 

He / She / It + HAD NOT BEEN + working / eating 

QUESTION:  HOW LONG + HAD + I / They / We / You + BEEN + working / eating ?  HOW LONG + HAD + He / She / It + BEEN + working / eating ?  For Example,

She had been working at that company for a year when she met him. 

The children had been playing and so the room was a mess! 

We had been living in Spain for three months when we had to leave. 

We had been having our meal for thirty minutes when the guests arrived.

USES 

The past perfect continuous is a verb tense that indicates something that began in the past, continued in the past, and also ended at a defined point in the past. 

For example,

He had been drinking milk when Mom walked into the kitchen. 

I had been working at the company for five years when I got the promotion. 

We had been cleaning the house for an hour when the kids arrived. 

When, for, since, and before are words that you may see used alongside the past perfect continuous tense. 

For example,

She had been walking three miles a day before she broke her leg. 

The program that was terminated had been working well since 1990. 

Ted had been playing the piano for 25 years when she was finally asked to do a solo performance.