16. They have bought some fruit juice.
-> Some fruit juice have been bought
17. "I must go to Ha Noi tomorrow." said Tuan.
-> Tuan said he must go to Ha Noi the next day
18. They last talked to their mother about their work last month.
-> They haven't talked to their mother about their work since last month
19. Jim is more hard-working than Susan.
-> Susan is not as hard-working as Jim