The past perfect continuous tense describes a continuous action that started in the past and continued up until another time in the past. It is formed with had been + present participle (verb+ing). It is used to show the duration of an action before something else in the past (e.g. "They had been talking for over an hour before Tony arrived") or to explain the cause of something in the past due to a previous ongoing action (e.g. "He was tired because he had been exercising hard"). Non-continuous verbs and certain mixed verb meanings cannot be used in the past perfect continuous tense.