Тест по английскому, будущее время, пояснить почему

1. The box must be taken to the station.
2. The river can be crossed by you on a raft.
3. The building of the house can be finished by the workers very soon.
4. The debts must be paid by you the day after tomorrow.
5. Three of these exercises must be done by you tomorrow.
6. The book you need can be found by you in any library.
7. These letters must be sent by us at once.
8. This mistake can be easily forgiven by me.
9. Such berries can be found by you everywhere.
10. This work must be done by you carefully.
11. They are said by the doctor that she must be taken to the hospital.
12. The work can be done by you in three days.
13. All the library books must be returned by students before each summer vacation.
14. Even the tallest trees can be climbed by monkeys.
15. All your money mustn't be spent by you at once.
16. His advice can't be taken by us.