Past perfect simple = I had worked                              Past perfect continuous = I had been working

We use the past perfect simple with action verbs to emphasise the completion of an event. We use the past perfect continuous to show that an event or action in the past was still continuing.

We use the past perfect simple to refer to the completion of an activity and the past perfect continuous to focus on the activity and duration of the activity.

The past perfect simple suggests something more permanent than the past perfect continuous, which can imply that something is temporary.

Some verbs are not used very often in the continuous form.

We don’t use the continuous form with some verbs of mental process (know, like, understand, believe) and verbs of the senses (hear, smell, taste):

We’d known for a long time that the company was going to close.

Not: We’d been knowing …

We’d tasted the milk and had decided it was bad, so we threw it away.

Not: We’d been tasting the milk …

We don’t use the continuous form with actions that are completed at a single point in time (start, stop):

Had they started the game on time?

Not: Had they been starting the game on time?