https://en.wiktionary.org/wiki/past_perfect_continuous
past perfect continuous - Wiktionary, the free dictionary
past perfect continuousthe freewiktionarydictionary
https://perfect-english-grammar3.teachable.com/courses/terrific-tenses/lectures/13230066
Past continuous quiz | Perfect English Grammar
Advanced level review of all the English tenses
past continuousquizperfectenglishgrammar