When do we use Past Perfect Continuous?
Duration and process before a past moment
Past Perfect Continuous describes an activity that was in progress for a period of time BEFORE a specific past moment. It emphasises the DURATION of the activity and often explains the cause of a past result.
How long an activity had been going on before something else happened.
Nora had been cooking for three hours when the judges arrived.
"for" + duration before judges arrived
She had been practising since January before the competition started.
"since" + starting point
By the time he arrived, we had been waiting for an hour.
"by the time" + duration
The ongoing activity explains WHY something was true at a past moment.
Her hands were sore because she had been chopping vegetables all day.
chopping caused sore hands
He was tired because he had been working since 6 am.
working caused tiredness
The kitchen smelled wonderful because she had been baking.
baking caused the smell
Past Perfect Continuous = DURATION/PROCESS before a past moment. Past Perfect = COMPLETED action before a past moment. "She had been cooking for 3 hours." (process) vs "She had cooked the meal." (completed result)