1.What were you doing when I called you last night?
2. I was sitting in a café when you called.
3. When you arrived at the party, who was there?
4. Susie was watching a film when she heard the noise.
5. Yesterday I went to the library, next I had a swim, later I met Julie for a coffee.
6. We were playing tennis when John hurt his ankle.
7. What were they doing at 10pm last night? It was really noisy.
8. He was taking a shower when the telephone rang.
9. He was in the shower when the telephone rang.
10. When I walked into the room, everyone was working.
11. It was a day last September. The sun was shining, the birds were singing. I was walking along the street when I met an old friend.
12. He was living in Russia when the Revolution started.
13. When her train got to the station, we were waiting on the platform.
14. He was so annoying! He always left his things everywhere.
15. On holiday we visited Rome, saw the Vatican, and spent a few days at the beach.
Past Continuous употребляется, когда идет речь о длительном действии в прошлом, умеющим место в определенный момент. (I was reading a book when mother came). Либо, когда есть ссылка на конкретное время в прошлом (10 o'clock yesterday и т.п.)