The document discusses the past perfect progressive tense. It is used to describe an action that started in the past and continued up until another time in the past. Some key points: - It shows something that started in the past and continued up until another time point in the past. - Common verbs used with the past perfect progressive tense are "watching", "waiting", "crying". - It should not be used with stative verbs like "know", "understand", "owe". - The past perfect progressive expresses one ongoing past activity that happened before another past action/event.