Past Perfect

Formula:

had + Past Participle (V3)

For example: had finished

Subject + had + past participle (V3).

Example 1 (sequence of actions):

She had left the office when I called her.

Example 2 (with by the time):

By the time we arrived, the film had already started.

Example 3 (explaining reason):

He was tired because he had worked all day.

Past Perfect vs Simple Past

Simple Past describes individual past actions. Past Perfect shows that one event occurred before another past event.

Example: I arrived at 8pm. She had left before I arrived.

Past Perfect vs Past Perfect Continuous

Past Perfect emphasizes the completion; Past Perfect Continuous emphasizes the duration of the action until a past moment.

Example: They had finished the game. They had been playing for two hours.

How to choose:

If you need to show the order of events — use Past Perfect.
If you need to show the duration up to a past moment — use Past Perfect Continuous.