PAST PERFECT CONTINUOUS

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

Structure of past perfect continuous

I'd (I had) been working hard. affirmative form
I hadn't (had not) been working hard. negative form
Had you been working hard? interrogative form