I wrote "Ο ελέφαντας είναι τεράστιο ζώο" and was marked wrong. If the article can be absent in many cases, why is it necessary here?
Yes, agreed, my understanding is that indefinite articles really only need to be used if there could be any confusion over quantity and can be dropped otherwise. Reported!
It's not necessary. I just added it, thank you for your comment! ^.^
Many thanks once again!
If ελέφαντας is a masculine noun (ο ελέφαντας) why is τεράστιο right, instead of τεράστιος?
Because the adjective is describing ζώο, animal, not the elephant itself (at least not directly). We could say "ο ελέφαντας είναι τεράστιος", as then the adjective is directly applied to the elephant.