Here are the correct verb forms:
1. Last night, when I arrived, they were having dinner.
2. Nam has not bought a new computer since he sold the old one.
3. Yesterday Tuan asked me to go to the station with him, but I did not want to leave because I was waiting for a phone call.
4. I met him in the park yesterday afternoon. He was reading a newspaper.
5. Helen and Jack were playing games online when I knocked on the door.
6. Last night while I was watching TV, my sister was doing a crossword.
7. My father will go home as soon as he finishes his work this afternoon.
8. I am sure before she comes tomorrow, we will have got everything ready.
9. When you see him tomorrow, you will recognize him.
10. The old man fell as he was getting into the bus.
11. He will write to me when he arrives in Hanoi tomorrow.
12. When I saw him yesterday, he was sitting asleep in a chair.
13. Mr. Brown will phone me as soon as he finishes his work.
14. The light went out when we were studying our lesson last night.
15. Whatever job you decide to do in the future, I will support you.
16. I can’t come to your party next Friday as I will be working on that day.
17. The mailman came while the Le family were sleeping.
18. They have not spoken to each other since they quarreled with each other.
19. When we came, they were planting some trees in the garden.
20. I will be waiting at the bus stop when you arrive.
21. Since I last saw you, I have been ill.
22. Last night, while their parents were watching TV, their children were running about.
23. Why didn’t you listen while I was speaking to you?
24. I hope it won't rain when we start early tomorrow.
25. Since I moved here last year, I have worked / have been working for a bank.
26. All the students were singing a merry song when I arrived.
27. He was sleeping, so he did not understand what you said to him.
28. You must tell me what you have done since I last saw you.
29. Last night, I was practicing the piano while my brother was reading a book.
30. This morning when the clock went off, I left the bed immediately.
31. I was doing the washing up when the phone rang.
32. When Mrs. Brown looked out, she saw her children playing near a pool.
33. Simon got lost because he had not taken a map with him.
34. What were you doing when the mailman came to your house this morning?
35. Julia has made over 30 movies since she moved to Paris.
36. My brother found the Old Friends website while he was surfing the internet.
37. This happened to Peter when he was working in the library.
38. My father has felt better since he moved to this town.
39. I will tell you the latest news as soon as I come back.
40. I have known her since I moved to this neighborhood.
41. I knew him when I worked in this factory.
42. She will be watching TV when she gets there tomorrow.
43. Were you watching TV as I phoned you last night?
44. They looked at the balloon until it disappeared.
45. Since I started learning English, I have had three different teachers.
46. Yesterday afternoon while I was watering the garden, it began to rain.
47. I will wait for her until she finishes her work.
48. Last night, when I saw him on Le Duan Street, he ran away.
49. As she leaves home tonight, she will call you.

1. arrived / were having
2. has not bought / sold
3. asked / did not want / was waiting
4. met / was reading
5. were playing
6. was watching / was doing
7. will go / finishes
8. comes
9. see / will recognize
10. fell / was getting
11. arrives
12. saw / was sitting
13. will phone
14. went / were studying
15. decide / will support
16. am working (hoặc will be working)
17. came / was sleeping
18. have not spoken / quarreled
19. came / were planting
20. will be waiting
21. saw / have been
22. were watching / were running
23. was speaking
24. will not be raining / start
25. moved / have worked
26. were singing
27. was sleeping
28. have done / saw
29. was practicing / was reading
30. went / left
31. was doing
32. saw
33. got / had not taken
34. were... doing / came
35. has made
36. was surfing
37. happened
38. moved
39. will tell
40. have known
41. knew
42. will be watching / gets
43. Were... watching / phoned
44. disappeared
45. started / have had
46. was watering / began
47. finishes
48. saw / ran
49. leaves / will call

Đáp án:

1. arrived – were having
2. has not bought – sold
3. asked – didn't want – was waiting
4. met – was reading
5. were playing
6. was watching – was doing
7. will go – finishes
8. comes
9. see – will recognize
10. fell – was getting
11. arrives
12. saw – was sitting
13. will phone
14. went – were studying
15. decide – will support
16. will be working
17. came – were sleeping
18. have not spoken – quarreled
19. came – were planting
20. will be waiting
21. saw – have been
22. were watching – were running
23. was speaking
24. will not be raining – start
25. moved – have worked
26. were singing
27. was sleeping
28. have done – saw
29. was practicing – was reading
30. went – left
31. was doing
32. saw
33. got – had not taken
34. were you doing – came
35. has made
36. was surfing
37. happened
38. moved
39. will tell
40. have known
41. knew
42. will be watching – gets
43. Were you watching – phoned
44. disappeared
45. started – have had
46. was watering – began
47. finishes
48. saw – ran
49. leaves – will call

`***` Giải thích:

1. Last night, when I arrived, they were having dinner.
2. Nam has not bought a new computer since he sold the old one.
3. Yesterday Tuan asked me to go to the station with him, but I did not want to leave because I was waiting for a phone call.
4. I met him in the park yesterday afternoon. He was reading a newspaper.
5. Helen and Jack were playing games online when I looked on the door.
6. Last night while I was watching TV, my sister was doing crossword.
7. My father went home as soon as he finished his work this afternoon.
8. I am sure before she comes tomorrow, we will have got everything ready.
9. When you see him tomorrow, you will recognize him. At that time, he will be wearing a yellow shirt.
10. The old man fell as he was getting into the bus.
11. He will write to me when he arrives in Hanoi tomorrow.
12. When I saw him yesterday, he was sitting asleep in a chair.
13. Mr. Brown will phone me as soon as he finishes his work.
14. The light went out when we were studying our lesson last night.
15. Whatever job you decide to do in the future, I will support you.
16. I can’t come to your party next Friday as I will be working on that day.
17. The mailman came while the Le family was sleeping.
18. They have not spoken to each other since they quarrelled each other.
19. When we came, they were planting some trees in the garden.
20. I will be waiting at the bus stop when you arrive.
21. Since I last saw you, I have been ill.
22. Last night, while their parents were watching TV, their children were running about.
23. Why didn’t you listen while I was speaking to you?
24. I hope it will not rain when we start early tomorrow.
25. Since I moved here last year, I have been working for a bank.
26. All the students were singing a merry song when I arrived.
27. He was sleeping so he did not understand what you said to him.
28. You must tell me what you have been doing since I last saw you.
29. Last night, I was practicing the piano while my brother was reading a book.
30. This morning when the clock went off, I left the bed immediately.
31. I was doing the washing up when the phone rang.
32. When Mrs. Brown looked out, she saw her children playing near a pool.
33. Simon got lost because he did not take a map with him.
34. What were you doing when the mailman came to your house this morning?
35. Julia has made over 30 movies since she moved to Paris.
36. My brother found the Old Friends website while he was surfing the internet.
37. This happened to Peter when he was working in the library.
38. My father has felt better since he moved to this town.
39. I will tell you the latest news as soon as I come back.
40. I have known her since I moved to this neighborhood.
41. I knew him when I worked in this factory.
42. She will be watching TV when she gets there tomorrow.
43. Were you watching TV as I phoned you last night?
44. They looked at the balloon until it disappeared.
45. Since I started learning English, I have had three different teachers.
46. Yesterday afternoon while I was watering the garden, it began to rain.
47. I will wait for her until she finishes her work.
48. Last night, when I saw him on Le Duan street, he was running away.
49. As she leaves home tonight, she will call you.