18. Tom said that he was decorating their house then. 

19. She asked Hoa if she had come to the party the night before.

20. A new car was bought last week.

21. Because of the loud noise from the music club, the residents of the street cannot sleep. 

__

18+19: Câu gián tiếp:
- Lùi thì modal verbs: can -> could, will -> would, may -> might
- Lùi 1 thì của các V thường: HTĐ -> QKĐ, QKĐ -> QKHT, HTTD -> QKTD,...
- Chuyển ngôi các đại từ: he -> him, she -> her, tên người (tùy giới tính) -> he/she, you -> me/us,....
- Chuyển đổi các trạng từ: this -> that, here -> there, now -> then, tomorrow -> the next day/ the following day, ago -> before,...

20: Câu bị động thì QKĐ: S+was/were+V3/ed+(by O).

21: S+V, so+S+V -> Because of+N/N phrase, S+V.

`18.` Tom said that he was decorating their house then. 

- Thì HTHT `->` QKTD

- now `->` then

`19.` She asked Hoa if she had come to the party the night before.

- thì HTHT `->` QKHT

- Câu tường thuật dạng Yes or No questions

`20.` A new car was bought last week.

- Bị động thì QKD

- S + was/were + VpII +(by +O) + time

`21.` Because of the loud noise from the music club, the residents of the street cannot sleep.