10. They began organizing neighborhood clean-ups seven years ago.

• Giải thích: Ta chuyển từ "have organized" sang "began organizing" và thêm cụm từ chỉ thời gian "seven years ago".

11. How long has she been tutoring students after school?

• When did she begin tutoring students after school?
• Giải thích: Câu hỏi chuyển từ dạng "How long" sang "When" để hỏi về thời điểm bắt đầu.

12. They haven't organized a clothing drive for five months.

• The last time they organized a clothing drive was five months ago.
• Giải thích: Câu này đã ở dạng đúng rồi.

13. We began donating school supplies six years ago.

• Giải thích: Tương tự câu 10, ta chuyển từ "have donated" sang "began donating" và thêm cụm từ chỉ thời gian.

14. They began helping with the community garden four years ago.

• Giải thích: Tương tự câu 10 và 13.

15. They haven't visited the elderly home for two months.

• The last time they visited the elderly home was two months ago.
• Giải thích: Tương tự câu 12.

16. We haven't cleaned up the beach for three months.

• The last time we cleaned up the beach was three months ago.
• Giải thích: Tương tự câu 12.

17. How long have you been helping at the youth center?

• When did you begin helping at the youth center?
• Giải thích: Tương tự câu 11.

18. We haven't planted trees in the park for two seasons.

• The last time we planted trees in the park was two seasons ago.
• Giải thích: Tương tự câu 12.

19. We haven't helped with the community garden for a year.

• The last time we helped with the community garden was a year ago.
• Giải thích: Tương tự câu 12.

20. We have been cleaning up the riverbanks for three years.

• We began cleaning up the riverbanks three years ago.
• Giải thích: Tương tự câu 10.