Past Perfect and Past Perfect Continuous for ESL
https://www.thoughtco.com/past-perfect-and-past-perfect-continuous-121119301.03.2008 · Past Perfect Continuous Structure. Positive. Subject + had + been + verb + ing. I, You, He, She, We, They had been working for two hours when she telephoned. Negative. Subject + had + not (hadn't) + been + verb + ing. I, You, He, She, We, They hadn't been paying attention for long when he asked the question.