Question

глаголы в начале задания: can, cannot,may,must,mustn't,should.

Answer 1

1. must (но лучше бы have to)
2. may
3. had to (must не употребляется в прошедшем времени)
4. had to
5. could
6. can
7. had to
8. had to, couldn't
9. mustn't
10. may
11. can't
12. shouldn't
13. couldn't
14. needn't  
15. shouldn't
16. was able to (=could)
17. cannot
18. needn't
19. have to
20. must

1. must
2. can't
3. can't
4. must
5. shouldn't (needn't)