The past perfect continuous is formed using had + been + present participle. Questions are indicated by inverting the subject and had. Negatives are made with not.
Statement: She had been waiting there for more than two hours when her taxi finally arrived.
Question: Had she been waiting there for more than two hours when the taxi finally arrived?
Negative: She had not been waiting there for more than two hours when the taxi finally arrived. https://www.englishpage.com/verbpage/pastperfectcontinuous.html
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