What is an example of a sentence written in the past perfect progressive tense?
The example sentence can be:
The ground was wet yesterday, because it had been raining all previous night.
Past Perfect tense is used to indicate which of 2 past actions took place earlier. If we want to emphasize, that the action took long time we use progressive/continuous form.
In the above sentence had been raining is Past Perfect Progressive, while had rained would be Past Perfect (Simple) form.