21. Jim was absent because he was sick.
- Because he's sick.
22. We were late for our game time because of the long lines at the ticket window.
- Because there was a long queue at the ticket window.
23. I couldn't see anything because it was too dark.
- Because it's too dark.
24. Because the weather is warm and humid, the rice grows very well here.
- Because the weather is warm and humid.
25. Jane never said an unpleasant word to Tom because of her love for him.
- Because Jane cherishes tom's love for him.
26. No one can solve this math problem.
 This is very difficult.
27. They believe he will pass the final exam.
 He passed as expected.
28. She was too old to carry that heavy briefcase.
 She was like that because she had to carry that heavy briefcase.
29. He rarely keeps his promises.
 Rarely does he keep his promises.
30. I have never traveled by plane in my life.
 I've never traveled by plane in my life.
31. Jack is not old enough to watch horror movies.
 So did Jack, because he was too young.
32. You will catch a cold if you do not keep your feet dry.
 Unless keeping your body warm.
33. He was seen leaving the office yesterday.
 He left the office yesterday.
34. She was late because it rained.
 Since it rained, she was late.
35. "You better not lend them more money." Alice said.
 Alice advised me not to lend them any more money.
36. She decided to give him some money.
 She does what he asks for.
37. We must pay attention to the teacher's explanation.
 We must answer the questions of the assigned teachers.
38. This flower is very beautiful.
 What's more beautiful than this flower?
39. He spent 2 hours doing math exercises.
 It took 2 hours to do the math exercises.
40. They have to change the date to meet again.
 The date they met again.
41. It's too late to do anything.
 It's too late for anyone to do anything.
42. I don't go out because it rains.
 If it's sunny, I can go outside.
43. I have fewer books than she has.
 She has more books than I do.
44. Mary visited us three days ago.
 Mary paid the doctor for us.
45. The plumber fixed the leak this morning.
 They fixed the plumbing fixture this morning.
46. ​​Although he has an English name, he is actually German.
 Although the name is English, but the original is still German.
47. I have found a place to live but it is difficult.
 I have trouble finding a place to live.
48. The book was so good that I couldn't put it down.
 They wrote the book so good that I read it and couldn't put it down.
49. He's fat because he doesn't do any exercises.
 But if he practices he will have a good body.
50. Turn off all switches before leaving the factory.
 All switches have been turned off when they were melted.

Mình chưa chắc đây đã đúng đâu nên sai chỗ nào bạn thông cảm cho mình nha !!!

21. Jim was absent because he was ill .
- Because of illness , Jim was absent .

22. We were late for the play because of the long line at the ticket-window .
- Because there was a long line at the ticket-window , we were late for the play .

23. I couldn’t see anything because it was so dark .
- Because of the dark , i couldn't see anything .

24. Because the weather is warm and damp, rice grows very well here .
- Because of warm and damp weather, rice grows very well here .

25. Jane never says a hard word to Tom because of her love for him .
- Because Jane loves Tom , she never says hard .

26. Nobody could solve this mathematical problem .

- This mathermatical problem couldn't be solved .

27. They believe that he will pass the final exam .

- He is believed to pass the exam .

28. She was too old to carry that heavy case .

- She was so old that she can't carry that heavy case .

29. He seldom keeps his promise .

- Seldom does he keep his promise .

30. I have never travelled by plane in my life .

- Never have i travelled by plane in my life .

31. Jack is not old enough to see horror films .

- Jack is too young to see horror films .

32. You will catch cold if you don’t keep your feet dry .

- Unless you keep your feet dry , you will catch a cold .

33. People saw him leave the office yesterday .

-He was seen to have left the office yesterday .

34. She came late because of the rain .

- Because it was raining , she came late .

35. “ You’d better not lend them any more money . ” said Alice .

- Alice advised me not to lend them any more money .

36. She decided to help him with some money .

-  She made the decision to help him with some money .

37. We have to notice the teacher’s explanation .

- We have to pay attention to the teacher's explanation .

38. This flower is very beautiful .

- What a beautiful flower .

39. He spent 2 hours  doing the mathematical exercises .

- It took him 2 hours to do the mathematical exercises .

40. They have to change the date of the meeting again .

- The date of the meeting will have to be changed again .

41. It was so late that nothing could be done .

- It was too late for anything to be done .

42. I didn’t go out because it rained .

- If it hadn't , i would have gone .

43. I have fewer books than she has .

- She has more books than me .

44. Mary visited us three days ago .

- Mary paid a visit to us three days ago .

45. The plumber repaired the leak this morning .

- They had the leak been repaired by the plumber .

46. Although he’s got an English name, he is in fact German .

- Despite his English name, he is in fact German .

47. I found a place to live but it was difficult .

- I had difficult finding a place to live .

48. The book was so good that I couldn’t put it down .

- It was such a good book that i couldn't put it down .

49. He’s fat because he doesn’t take any exercises .

- But if he took some exercise , he wouldn't be fat .

50. Turn off all the switches before leaving the workshop .

- All the switches must be turned off before you leave the room .

CHÚC BẠN HỌC TỐT !