1. I (sit) on the bench for half an hour and then (begin) reading a book. 2. You (see) him this morning? Yes. He (stand) in the hall laughing. 3. He (come) in and (see) Nelly who (draw) a strange picture. 4. I (not understand) what Mr Green (do). 5. Nick and Fred (drive) along the road when the accident (happen). 6. When I (come in), she (set) the table. 7. The businessman (fly) to England yesterday. 8. I (not hear) what he (say). I (type) at the moment. 9. When it (happen)? It (happen) when you (talk) to your secretary. 10. We (walk) in silence for 5 minutes, then he (speak). 11. I (tell) him he should not read while he (eat). 12. The mouse (hide) while the cat (watch). 13. The train (strike) an obstacle on the line, but it (not stop) because it (travel) too fast at the time. 14. I (sit) in my garden yesterday, when the servant (come) and (say) that a visitor (wait) for me in the hall. 15. He (not like) to play while others (work). 16. Sorry, what you (say)? - I (not listen). 17. You (enjoy) your game of tennis? - No, I (not play) as it (rain). 18. Why you (tell) Ann her lessons (be) boring? - I (not mean) it. I only (joke). 19. Why you (make) so many mistakes in the last test? - I (think) about my boyfriend. 20. Just imagine! I (wait) for you for an hour yesterday and you (sleep) at that very time!

Решение:

Здесь нужно подставить правильную форму глагола в Past Indefinite (простое прошедшее время) или Past Continuous (прошедшее длительное время).

1. I sat on the bench for half an hour and then began reading a book. (Два законченных действия в прошлом).
2. You saw him this morning? Yes. He was standing in the hall laughing. (Past Continuous для действия, которое происходило в определенный момент в прошлом).
3. He came in and saw Nelly who was drawing a strange picture. (Past Continuous для длительного действия, которое прервалось или происходило параллельно).
4. I did not understand what Mr Green was doing. (Past Continuous для действия, происходившего в момент разговора).
5. Nick and Fred were driving along the road when the accident happened. (Past Continuous для длительного действия, которое было прервано другим действием в Past Indefinite).
6. When I came in, she was setting the table. (Past Continuous для действия, которое было в процессе, когда другое действие произошло).
7. The businessman was flying to England yesterday. (Past Continuous для действия, которое происходило в течение всего прошлого дня).
8. I did not hear what he was saying. (Past Continuous для действия, которое происходило в определенный момент в прошлом). I was typing at the moment. (Действие, которое было в процессе в момент речи).
9. When it happened? It was happening when you were talking to your secretary. (Past Continuous для действий, которые происходили одновременно и длились).
10. We walked in silence for 5 minutes, then he spoke. (Два последовательных действия в прошлом).
11. I told him he should not read while he was eating. (Past Continuous для фонового действия).
12. The mouse was hiding while the cat was watching. (Два параллельных длительных действия).
13. The train struck an obstacle on the line, but it did not stop because it was travelling too fast at the time. (Past Continuous для действия, которое было в процессе и объясняло другое действие).
14. I was sitting in my garden yesterday, when the servant came and said that a visitor was waiting for me in the hall. (Past Continuous для длительного действия, прерванного другим, и для действия, которое было в процессе).
15. He did not like to play while others were working. (Past Continuous для параллельных действий).
16. Sorry, what you were saying? - I did not listen. (Past Continuous для действия, которое было в процессе).
17. You enjoyed your game of tennis? - No, I was not playing as it was raining. (Past Continuous для длительного действия, которое было причиной другого действия).
18. Why you told Ann her lessons were boring? - I did not mean it. I only joked. (Past Indefinite для завершенных действий).
19. Why you made so many mistakes in the last test? - I was thinking about my boyfriend. (Past Continuous для объяснения причины).
20. Just imagine! I was waiting for you for an hour yesterday and you were sleeping at that very time! (Past Continuous для действий, которые происходили одновременно и длились в определенный момент в прошлом).

Ответ: sat, began; saw, was standing; came, saw, was drawing; did not understand, was doing; were driving, happened; came in, was setting; was flying; did not hear, was saying, was typing; happened, was happening, were talking; walked, spoke; told, was eating; was hiding, was watching; struck, did not stop, was travelling; was sitting, came, said, was waiting; did not like, were working; were saying, did not listen; enjoyed, was not playing, was raining; told, were, did not mean, joked; made, was thinking; was waiting, were sleeping.