10. They (play) chess when the lights (go) out.

10. They were playing chess when the lights went out.

Объяснение: Используем Past Continuous (were playing) для действия, которое длилось в определенный момент в прошлом, и Past Simple (went) для действия, которое произошло однократно во время первого действия.

Ответ: were playing, went