Ответ:

  1. I have got a ruler.
  2. I have got a green ruler.
  3. I have got a nice green ruler.
  4. I have got green rulers.
  5. Have you got a pencil?
  6. Have you got a red pencil?
  7. Have you got a long red pencil?
  8. Have you got a book?
  9. Have you got an English book?
  10. Have you got English books?
  11. Have you got a Russian book?
  12. Have you got an interesting Russian book?
  13. They are singers. I am a singer too.
  14. Are you a good singer?
  15. I would like to be a dancer.

Артикль a употребляется перед исчисляемым существительным в единственном числе, а an — перед гласным звуком; перед существительными во множественном числе артикль не ставится.