The document discusses the past perfect tense, which indicates that an action was completed at some point in the past before something else happened. It is formed using had plus the past participle of the verb. The past perfect refers to a time earlier than before now and is used to clarify that one past event occurred before another. It does not matter which event is mentioned first, as the tense makes clear the order. Examples of affirmative, negative, and interrogative sentences in the past perfect tense are provided.