Past perfect simple or past perfect continuous?

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.

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The builders had put up the scaffolding around the house.	Past perfect simple emphasises the completion of the action (the scaffolding is up).
The builders had been putting up the scaffolding when the roof fell in.	Past perfect continuous emphasises a continuing or ongoing action.

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.

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I’d waited an hour for the bus.	Past perfect simple emphasises the completion of the activity (the waiting is over).
I’d been waiting an hour for the bus.	Past perfect continuous focuses on the duration of the activity.

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

Compare

She’d always lived with her parents.	We don’t know how long.
She’d been living with her parents.	Suggests a temporary situation.

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?