Mình gửi bạn bài mình sửa nha, chúc bạn học tốt!
Exercise 6. Find and correct the mistakes.
1. Every day I go to school on foot.
2. She's on a bus to Sapa to visit her aunt.
3. What sources of alternative energy should we develop in our
country?
4. Jane is here. I think these sandals are hers. 
5. Is this John's phone? - Yes, it is mine.
6. One of (the) most important discoveries of the 19th century was a method
of using natural gas for cooking and heating. (Mình không chắc vì thường so sánh nhất phải có mạo từ the)
7. We are willing to join this activity to help elders.
8. You should try to ride a boat. It's really fantastic.
9. Their school will hold the school festival next weekend.
10. Will cars use solar power instead of gas by the year 2025?