11- He ................................. (fix) his bike when it broke down.

Для образования Past Continuous необходимо использовать вспомогательный глагол to be в прошедшем времени (was/were) и основной глагол с окончанием -ing.

В данном случае подлежащее - He, следовательно, используем was.

fix + -ing = fixing

Получаем: He was fixing his bike when it broke down.

Ответ: He was fixing his bike when it broke down.