Past Perfect Continuous

We use the past perfect continuous tense when referring to an action that began in the past and was still happening when a different action started.

It is formed by using the helping verb had together with the linking verb been and the present participle of the verb (base verb + ing).

Example;

We had been driving for 10 minutes when the police stopped us.

Example;

She had been running the company for twenty years by the time she retired.

Or

By the time she retired, she had been running the company for twenty years.