Ответ:

1. When my father came home, I was having dinner.

2. When Alice returned, I was listening to the radio.

3. When mother entered his room, he was drawing a picture.

4. When my sister came in, I was doing my lessons.

5. When her aunt was airing the room, she caught cold.

6. When I met her, she was going to the office.

7. He was washing his face when somebody knocked at the door.

8. The young people were dancing when I came to the party.

9. When he washed the dishes, he broke a plate.

10. What they were doing, when you saw them?