✓ expresses an action that began in the past and was completed in the past before something else occurred.
Past tense of had + past participle = past perfect tense
Example - Paul tried to hide the vase because he had broken it.
past perfect phrase = had broken. There’s our past tense had and our past participle.
The sentence is describing something that began in the past and was completed before something else occurred. Paul broke the vase. The action is complete, therefore the tense is perfect, and it happened in the past, therefore the word had appears in our equation.