Past Perfect Continuous is used to talk about an event that happened in the past and continued happening until a second moment in the past. The action that came first is the past perfect (or, past of the past). We can identify the past perfect continuous by its structure.

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The structure for perfect forms is (have + verb in past participle), and structure for continuous form is (verb to be + verb -ing).

Adding these structures (have + [to be in past participle = been] + verb -ing).

Thus, the structure for past perfect continuous is (auxiliary verb 'have' in past = had + [to be in past participle = been] + verb -ing):

Had been + verb in -ING

She went to the doctor because she had been having terrible headaches.

Where had she been staying?

They had not been working for long before the results appeared.

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References:

AllGenius Drive

Cambridge

Exam English