The past perfect tense expresses an action in the past that ended before another action in the past. In short, it is “the past in the past” action.

Past Perfect timeline

One action happens before another in the past

SCENARIO:
You’re now at the bus station but the bus isn’t there anymore, so you ask the porter. He informs you that the bus is gone. He says:

• The bus has just left when you arrived. – [The porter is saying it now.]

Later, you go home and tell this to your mom:

• Mom, the porter said that the bus had just left when I arrived.

Use “had” followed by the past participle form of the verb when you write a sentence in the past perfect tense.