Can someone help me with these sentences?

Which of them are grammatically correct?
Thank you very much in advance.

1) They are not able to dance well.
2) They can't dance well
3) Are you able to play the piano?
4) Can you play the piano?
5) Were you able to find the book you lost?
6) Could you find the book you lost (mistake?)
7) The painting was stolen but returned to the museum a year later.
Harry Potter books hold records/set records as fastest selling books of all time.
8) The painting is protected very well now/is very well protected (?)
9) It is protected by a glass case.
10) Analyze the theme of morality as dealt with by Hardy in his novel Tess of the D’Urbervilles explaining the significance of its subtitle.