21. They haven't visited HCM city for 2 years (QKĐ -> HTHT, last -> have/has not + VPII, ago -> for)

22. You shouldn't eat junk food. (câu điều kiện loại 2 If I were you: nếu tôi là bạn --> câu lời khuyên)

23. Being an environmentalist, my grandfather understood the dangers of open waste burning. (câu rút gọn chủ động) 

24. Applying for ASEAN scholarship is more and more challenging for students. (dùng V-ing lên làm chủ ngữ) 

25. He has read books for two months. (began + V-ing = have/has + VPII)

26. You should apply your application form before September 15th. (it is advisible --> you should + V  ..)

27. Swimming everyday is a good way to keep fit (động từ lên làm chủ ngữ) 

28. I haven't met him for 4 months (last -> have/has not + VPII, ago -> for)