Past perfect continuous

((+)) Subject + HAD BEEN + [verb+ing] --> She had been walking with her mum. 

((-)) Subject + HAD + NOT + BEEN + [verb+ing]  --> She hadn't been walking with her mum. 

((?)) HAD + Subject + past participle ?  --> Had she been walking with her mum? 

We use the past perfect continuous to talk about temporary actions, but we use the past perfect simple to talk about longer or permanent situations.

We were exhausted because we had been walking all day.

She met a man who had lived in the city for 70 years.

The past perfect continuous emphasizes that an action continued, but the past perfect simple emphasizes that the action was completed.

I had been watching a zombie film, and my head was full of strange images.

I had finished my meal, and I was ready to go home.