Ответ:
1. Sandra IS CHOOSING a new dress in the next department at the moment.
2. The students WERE PLAYING football, when we came to the stadium.
3. What ARE you DOING now? — I AM DRIVING my car.
4. The auditor IS CHECKING the financial statements in the chief accountant's study.
5. Mary IS WRITING a test. Don't call her.
6. Don't disturb the director! He IS LOOKING through the materials for the meeting.
7. Nelly WAS WEARING a new dress, when we met her last night.
8. When we saw Peter at the cafeteria he WAS DRINKING coffee with Jack.
9. Where were you on Friday morning? Mr. Easton WAS CALLING you from 9 till 11.
10. Listen! The President IS SPEAKING.
11. While we WERE PREPARING our papers the secretary WAS TYPING our resumes.
12. We WERE DISCUSSING your catalogues at that time yesterday.