The document discusses the perfect progressive aspect of verbs, which refers to an ongoing action that is either completed or in progress. It describes the past perfect progressive, present perfect progressive, and future perfect progressive tenses. The past perfect progressive uses "had been" plus the present participle to describe a continuous action that was completed in the past. The present perfect progressive uses "has/have been" plus the present participle to describe an action that began in the past and continues to the present. The future perfect progressive uses "will have been" or "will have to be" plus the present participle to describe an action that will continue and be completed in the future.