15. While Nga's family was eating, the mailman came
16. We have not talked to him since we left highschool
17. He promises he will learn hard next semester
18. I am given a new book by my teacher
19. In the usa, millions of old car tires are thrown away every year.
20. The entrance examination will be held on  August 2nd.
21. We don't know where to go for our holidays
22. She was disappointed to hear about that
23.let open the window in order to always keep fresh air in.