1-c. I couldn't play football because I hadn't brought my boots.
2-e. When I went back to the shop they'd sold all the cheap trainers.
3-f. Helen was upset yesterday because I hadn't remember her birthday.
4-d. We got to the theatre after the play had started.
5-a. I didn't wait for you because I hadn't read your text message.
6-b. When I arrived at the airport the plane had just landed.