Take number 1-21 leave number 6

Here are the completed sentences:

1. Copper is $\boxed{\text{a}}$ useful metal.
2. He is not $\boxed{\text{an}}$ honorable man.
3. $\boxed{\text{An}}$ able man has not always a distinguished look.
4. $\boxed{\text{A}}$ reindeer is a native of Norway.
5. Honest men speak $\boxed{\text{the}}$ truth.
6. (Skipped as requested)
7. Do you see $\boxed{\text{the}}$ blue sky.
8. Varanasi is $\boxed{\text{a}}$ holy city.
9. Aladdin had $\boxed{\text{a}}$ wonderful lamp.
10. The world is $\boxed{\text{a}}$ happy place.
11. He returned after $\boxed{\text{an}}$ hour.
12. $\boxed{\text{The}}$ school will shortly close for the Puja holidays.
13. $\boxed{\text{The}}$ sun shines brightly.
14. I first met him $\boxed{\text{a}}$ year ago.
15. Yesterday $\boxed{\text{a}}$ European called at my office.
16. Sanskrit is $\boxed{\text{a}}$ difficult language.
17. $\boxed{\text{The}}$ Ganga is $\boxed{\text{a}}$ sacred river.
18. $\boxed{\text{The}}$ lion is $\boxed{\text{the}}$ king of beasts.
19. You are $\boxed{\text{a}}$ fool to say that.
20. French is $\boxed{\text{an}}$ easy language.
21. Who is $\boxed{\text{the}}$ girl sitting there?