Final answer:
The answers to the Modals questions are: 17. must not, 18. wasn't able to, 19. didn't have to, 20. had to.
Step-by-step explanation:
17. Visitors must not stay in the hospital after ten pm.
18. We had an appointment yesterday afternoon but he wasn't able to see me.
19. I didn't have to ring her up for she did it herself.
20. The rain was so sudden that everybody had to take a shelter.
Modals:
17. The correct answer is must not because it is specifying that visitors are not allowed to stay in the hospital after ten pm.
18. The correct answer is wasn't able to because it indicates that he was unable to see you during the appointment.
19. The correct answer is didn't have to because it shows that you were not required to ring her up, as she did it herself.
20. The correct answer is had to because it highlights the necessity for everyone to seek shelter due to the sudden rain.