The past perfect continuous tense is used to describe an action that started in the past, continued for a period of time, and was completed before another past action.

Positive: subject + had been + verb-ing
Negative: subject + had not been + verb-ing
Question: Had + subject + been + verb-ing?

Common uses:

1. To describe a duration of time before a past action

• I had been studying for two hours before I took a break.
• She had not been exercising regularly before she injured herself.
• Had they been working on the project for a long time?

2. To describe a cause of a past action

• He was tired because he had been running for an hour.
• They were late because they had not been planning ahead.
• Had she been feeling sick before she went to the doctor?

3. To describe an ongoing action that was interrupted by another past action

• I had been cooking dinner when the phone rang.
• He had not been studying when it was time for the test.
• Had they been playing football when it started raining?