What is the structure used to form the past perfect progressive tense?

The past perfect progressive tense is formed using "had been" followed by the present participle of the verb. This structure indicates an action that was ongoing in the past and was completed before another action took place. For example, in the sentence "She had been studying for hours before the exam started," the phrase "had been studying" shows that the studying was happening continuously in the past up until another past event, which is the exam starting. This tense is particularly useful to convey the duration of an action leading up to a specific point in the past.