A sentence in Past Perfect Continues tells two things
(a) Action of past (denoted by verb) is not completed. It was going on at that moment
(b) ‘Starting time of action / work’ or ‘period of time’ during which action was taking place is given.
1) 1st form of verb is used.
2) “ing” is added with the first form of verb .
3) Helping verbà ‘had been’ is used with all Subjects.
4) Before time “since/for” is used (see the correct uses of Since/for). “Since” is used if time of beginning of action is given and “for” is used if “period of time” is given.
Affirmativeà Subject + had been + verb + object.
Negative à Subject + had not been + verb + Object.
Interrogative à Had + subject + been + verb + object?
Negative + Interrogative à Had + Subject + not been + verb + Object?
1) He had been working here since 1959.
2) She had not been sleeping for 2 hours.
3) Had they been swimming for long time?
4) Had you not trying to climb on the tree for half an hour.
5) I had been making this project since Monday.
For practice see