The Past Perfect tense is used to talk about actions that were completed before something else in the past. It is formed using "had" plus the past participle of the main verb. There are two main uses - to refer to a completed action that happened before another past action, and to indicate that something started in the past and continued up until another past time or event. Specific time words can be used with the Past Perfect, but are not always necessary. It is sometimes possible to use either the Past Perfect or simple past tense.