Here are the complete sentences using the past continuous:

1. Mary was playing the piano while her parents were watching TV.
2. She was painting a picture while her brother was doing his homework.
3. Jane was baking a cake while her father was decorating the house.
4. They were exploring the city streets while taking photographs all day.
5. My sister was playing badminton while my brother was flying a kite.
6. Ngoc was having a meeting at 9 a.m. last Wednesday.
7. Quang was reading a science book all morning.
8. Was he chatting with friends online at 10 p.m. yesterday?
9. I was saving money for months to buy a new car.
10. John was singing while his friends were dancing at the party.
11. They were studying Spanish all year to prepare for their trip.
12. My brother was practicing guitar all month to improve his skills.