21. He is a good student. He always does all the exams well because he is a good student.

22. Peter still went camping though it was snowing heavily. Even though it was snowing heavily, Peter still went camping.

23. Ann is very tired, however, she helps her mother with the housework. Even though Ann is very tired, she helps her mother with the housework.

24. Hoa was sick, so she missed all of the exams last week. (therefore) Therefore, Hoa missed all of the exams last week.

25. Tuan wanted to reduce air pollution. He planted more trees. (SO) So, Tuan planted more trees.

26. Mark is American, but he plays Vietnamese folk games well. Although Mark is American, he plays Vietnamese folk games well.

27. Although the girl worked hard, her stepmother wasn't happy. The girl worked hard, although her stepmother wasn't happy.

28. The boy is young, so he can't drive a car Because of his young age, the boy can't drive a car.

29. It rained heavily, so the main part of the festival was delayed for an hour. Because it rained heavily, the main part of the festival was delayed for an hour.

30. Duong was very busy, however, he spent the whole day at the Lim festival. Duong, although very busy, spent the whole day at the Lim festival.

31. Jack has a new laptop; however, he never logs on to his account on the Facebook Although Jack has a new laptop, he never logs on to his account on the Facebook.

32. I really enjoyed the concert. The music was too loud. Although the music was too loud, I really enjoyed the concert.

33. Mai is the head of the music club, so she knows the members very well. → Because of being the head of the music club, she knows the members very well.

34. The teacher punished this boy because he was very lazy. This boy was punished by the teacher because he was very lazy.

35. This boy was very lazy This boy was very lazy, so the teacher punished him.

36. It rained heavily. He arrived at the meeting on time Although it rained heavily, he arrived at the meeting on time.

37. She studied very hard, so she passed the exam. Because she studied very hard, she passed the exam.

38. I have a lot of homework tonight. I can't go to the movie theatre with you. (so) So, I can't go to the movie theatre with you.

39. Tom felt tired, so he went to bed earlier than usual Because of feeling tired, he went to bed earlier than usual.

40. Please increase the air-conditioner's temperature. We will be frozen in here. (otherwise) Otherwise, we will be frozen in here.

41. She wanted to go to the party, but she couldn't choose a suitable dress Although she wanted to go to the party, she couldn't choose a suitable dress.

1. He is a good student. He always does well because of his hard work.
   (Explanation: "does" should be used here for the verb "do" in the present tense.)

2. Peter won't go camping, even though it was snowing heavily.
   (Explanation: "won't" should be "will not" for the correct future form. "Even though" is a better phrase than "though" alone.)

22A. She was very tired; however, she helped her mother with the housework even though.
(Explanation: "However" should be used to contrast the two clauses. "Even though" connects the condition with the result.)

1. She missed a lot of the exams last week. Therefore, she had to retake them.
   (Explanation: "Therefore" is used to explain the consequence of her missing the exams.)

2. Tan wanted to reduce air pollution, so he planted more trees.
   (Explanation: "So" is used to indicate purpose or result.)

3. Mark is American, but he plays Vietnamese folk games well.
   (Explanation: "But" is used here to show contrast.)

4. Although the girl worked hard, her stepmother wasn't happy. The girl did not make her stepmother happy even though she worked hard.
   (Explanation: The sentence can be rewritten for clarity and to match the structure.)

5. The boy is young, so he can't drive a car because of his age.
   (Explanation: "So" shows cause and effect here.)

6. It rained heavily, so the main part of the festival was delayed for an hour.
   (Explanation: "So" is used to indicate a cause-effect relationship.)

7. Duong was very busy; however, he spent the whole day at the festival.
   (Explanation: "However" is used to contrast his busyness with the action of attending the festival.)

8. Jack has a new laptop; however, he never logs on to his account on Facebook.
   (Explanation: "Logs on" is the correct phrase.)

9. I really enjoyed the concert, although the music was too loud.
   (Explanation: "Although" is used to contrast the enjoyment with the loud music.)

10. Mai is the head of the music club, so she knows the members very well.
    (Explanation: "So" indicates the result of her position as head of the club.)

11. The teacher punished this boy because he was very lazy.
    (Explanation: This sentence is correct; no change needed.)

12. This boy was very lazy, so the teacher punished him.
    (Explanation: The effect of laziness is shown with "so.")

13. It rained heavily, but he arrived at the meeting on time.
    (Explanation: "But" is used to contrast the heavy rain with his timely arrival.)

14. She studied very hard, so she passed the exam.
    (Explanation: "So" indicates cause and effect.)

15. I have a lot of homework tonight, so I can't go to the movie theater with you.
    (Explanation: "So" is used to express the result of having homework.)

16. Tom felt tired, so he went to bed earlier than usual.
    (Explanation: "So" is used to show the result of his tiredness.)

17. Please increase the air-conditioner's temperature; we will be frozen in here otherwise.
    (Explanation: "Otherwise" is used to show the result of not increasing the temperature.)

18. She wanted to go to the party, but she couldn't choose a suitable dress.
    (Explanation: "But" shows the contrast between her desire and the obstacle.)