Past Perfect Simple

Past Perfect is used to express an action that occurred before another one in the past. For instance, check this example:
When I arrived to the station, the train had already left. 
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There are time expressions that are used with this tenses: for, since, just, when, never, until,etc. 

After analysing this, are you able to tell us the formula of this verb tense? 

had + past participle

Now, watch this video: