11. Izmir **is** a city.
12. He **is** a postman.
13. It **is** nine o'clock.
14. Manhattan **is** an island.
15. Mr. Richards **is** a lawyer.
16. I **am** ill. I **am** happy.
17. London **is** a big city.
18. Dave and Adrian **are** sisters. They **aren't** brothers.
19. New York **is** near to New Jersey.
20. Susan and I **are** teachers. We **aren't** students.
In these sentences, "is" is used with singular subjects (Izmir, He, It, Manhattan, Mr. Richards, London, and New York), and "are" is used with plural subjects (Dave and Adrian, Susan and I). "Am" is used for the first person singular (I) in sentences 16 and 20 to indicate a state of being. "Aren't" is the contraction of "are not" used for negation in the sentences where the subjects are plural. The choice of "isn't" or "am not" depends on the subject and context. The sentences are grammatically correct and follow the rules of subject-verb agreement.
In summary , the sentences correctly use "is" with singular subjects and "are" with plural subjects. "Am" and "aren't" are appropriately used for the first person singular and plural subjects, respectively.