15. Keeping the environment clean is very important. 

⇒ It’s very important to keep the environment clean.

16. They will show the time machine to the public when they finish it. 

⇒ The time machine will be shown to the public when it is finished.

17. He was delighted to receive his aunt’s letter.

⇒ He was delighted that his aunt sent him a letter.

18. Old car tires are recycled to make shoes and sandals. 

⇒ People recycle old car tires to make shoes and sandals.

19.I’m very sad that I wasn’t accepted in that group. 

⇒ I’m very sad not to be accepted in that group.

20.This is the best computer I have ever used.

⇒ I have never used the better computer than this before.

21.He has not chatted with his classmate through the internet for ages. 

⇒ It's ages since he last chatted with his classmates through the internet.

22.Trung's parents gave him a microcomputer on his birthday.

⇒ Trung was given a microcomputer on his birthday.

15. It's very important to keep the environment clean. ( It's very important to do something = Nó thì rất quan trọng để làm một việc gì đó )

16. The time machine will be shown to the public when they finish it. ( Câu bị động ở thì tương lai đơn : S + will + be + V(PII) + place + (by + O) + time )

17. He was delighted that his aunt sent him a letter. ( CT : S + tobe + adj/quá khứ phân từ + mệnh đề danh ngữ )

18. People recycle old car tires to make shoes and sandals. ( Chuyển từ câu bị động sang chủ động ở thì hiện tại đơn )

19. I'm very sad not to be accepted in that group. ( CT : S + tobe + adj + (not) + to V(inf) )

20. I have never used such a good computer before. ( Thì hiện tại hoàn thành : S + have/has + V(PII) + O )

21. It's age since he last chatted with his classmates through the internet. ( CT : It + tobe + khoảng thời gian/ mốc thời gian/.. + since + S + (last) + V(quá khứ đơn)

22. Trung was given a microcomputer on his birthday by his parents. ( Bị động ở thì quá khứ đơn : S + was/were + V(PII) + place + (by + O) + time )