14. It's a place where you can enjoy blue water and beautiful mountains. (改为同义句) 

It's a place                 blue water and beautiful mountains.

13. [2014 •重庆A ] Cao Hua swims fastest in our group.(改为否定句)

Cao Hua              fastest in our group.

12. He told me,   " Don't open the present until Christmas Day." (改为同义句)

 He told me                  the present until Christmas Day.

11. The students have a trip to the Great Wall once a year. (对画线部分提问)

                    the students have a trip to the Great Wall?

10. [2014 •黔西南〗I really don't know what I can do to help her out.(改为简单句)

I really don't know what               to help her out.

9. New York is a wonderful city. It never sleeps. (合并为一句)

New York is a wonderful city             

8. Please tell me which one I should buy for my kids. (改为同义句) 

Please tell me which one                 for my kids.

7. To understand their accents is really difficult. (改为同义句)

          really difficult           their accents.

6. We will come back in a month. (对画线部分提问) 

            will you come back?

5. Were they seen to get off the bus by their teacher yesterday? (改为主动语态)       their teacher                 off the bus yesterday?