Ответ:
1. Did they have their breakfast at the university dining-room?
They didn't have their breakfast at the university dining-room.
2. Did he do the translation without any difficulty?
He didn't do the translation without any difficulty.
3. Did you ski in the country yesterday?
I didn't ski in the country yesterday.
4. Did Agatha Christie write a lot of detective stories?
She didn't write a lot of detective stories.
5. Did my father buy a new adventure story yesterday?
My father didn't buy a new adventure story yesterday.
18them
19could have
20didn't understand
21mice
22would stay
23worse
24didn't hear
25was made
26will keep
1) singing
2) playing
3) washing
4) going
5) driving
6) feeding