答案：1. couldn’t 2. may 3. mustn’t 4. should 5. shall 6. needn’t
7. Must 8. Can; can’t; must 9. would 10. Ought 11. must 12. shall 13.must have seen
14. must be talking 15. might/might have missed 16. May 17. needn’t have told
18. should have 19. can’t be working 20. shouldn’t have given
