Past Perfect Progressive (Past Perfect Continuous)

Definition:

The past perfect progressive puts emphasis on the course or duration of an action taking place before a certain time in the past.

Form :

A: He had been talking.
N: He had not been talking.
Q: Had he been talking?

Use  :

action taking place before a certain time in the past
sometimes interchangeable with past perfect simple
puts emphasis on the course or duration of an action

signal words :

for, since, the whole day, all day