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