10. Denis (lose) his keys when he (be) on a bus.

Denis lost his keys when he was on a bus.
Explanation: The verb 'lose' is an irregular verb. In Past Simple, 'lose' becomes 'lost'. The verb 'be' is an irregular verb. In Past Simple, 'be' becomes 'was' (for singular subjects like Denis) or 'were' (for plural subjects). Since the subject is 'he' (Denis, singular), the correct form is 'was'.