I want to know why we can't use the past perfect progressive in the following sentence, as the answer key suggests the past perfect simple only, or maybe wee can!? Walt Disney _________________ (produce) cartoons for nearly 40 years before he died in 1966. The answer is in the use of the conjunction before. It suggests that something was complete prior to something else. Compare: He had been studying for three hours when the lights went out. - or - Walt had been producing his new movie when he died. The continuous/progressive is interrupted by the other. In the sentence you provided, it provides a history, but one does not follow, flow from or interrupt the other.