Study materials
Both sentences explain what happened before a past moment, but in different ways.
She was tired because she had been working all day.
Duration matters.
She was proud because she had finished the report.
The result matters.
This article explains how the Past Perfect Continuous is formed, when it shows duration and the trace of an action, how it differs from the Past Perfect, and why it is not used with state verbs.
Duration up to a past point
When what matters is not the earlier event itself but how long it went on, you use the Past Perfect Continuous, had been with an -ing verb. For or since often stands next to it. The negative uses hadn't been, and a question about duration starts with how long: How long had you been waiting? This form also explains a trace that could be seen in the past.
They had been working on it since six.
She was out of breath because she had been running.
Compare the two forms in one situation. The Past Perfect shows a result you can count. The Past Perfect Continuous shows a process and how long it lasted.
He had written three reports.
A result.
He had been writing reports all night.
A process.
State verbs are not used in this form. The Past Perfect is used instead: They had known each other for years.
Had done or had been doing
The pair works the same way as at the present point. Had done names a result that was ready at that moment. Had been doing says how long the action had gone on before it, and often explains where a state came from.
By lunch, Oksana had answered forty emails.
A result.
Oksana was exhausted. She had been answering emails since seven.
A duration.
The ground was wet. It had been raining all night.
A trace of the activity.
State verbs keep the simple form here too: We had known each other for years, not had been knowing.
Common mistakes
The first mistake is using the Past Perfect Continuous where a number is involved.
He had been writing three reports. → He had written three reports.
Three reports is a result, not a duration.
The second mistake is the continuous form of a state verb.
They had been knowing each other for years. → They had known each other for years.
Know describes a state.
The third mistake is since before a length of time.
We had been waiting since two hours. → We had been waiting for two hours.
For introduces a length of time.
The first mistake shows in the number. If the sentence names a quantity such as three reports, the Past Perfect is needed. The second shows in the verb itself. Know, like, own and other state verbs have no continuous form. The third shows after since. Since introduces a point in time such as six o'clock or Monday, and a length of time such as two hours takes for.