B3

1. was preparing
2. were playing
3. was practising
4. were sitting
5. were playing
6. was watching
7. was watching/ was reading / was doing / was playing
8. was doing
9. was watering 
10. were visiting 
11. were learning
12. were doing

m làm theo thứ tự trên xuống

1.Yesterday at six I was preparing dinner.
2.The kids were playing in the garden when it suddenly began to rain.
3.I was practicing  the guitar when he came home.
6.Most of the time we were sitting in the park.
7.At this time yesterday we were playing tennis in the schoolyard.
8.At 7 p.m yesterday he was watching TV.
9.Yesterday while my father was watching TV, my mother was reading the newspaper, my sister was doing her homework and I were playing chess with my friend.

III

1. Long began working in this factory 4 years ago
2. They started swimming in this river at 4.00 pm
3. Hoa has cooked meals for an hour
4. LAn has built this house since June
5. My father started playing tennis 30 minutes ago
6. we have listened to music since 7.00 this evening