Can't we use έναν here?
Nope. Έναν is for masculine nouns. Το αντικείμενο is neuter, so you'd have to use ένα. ^.^
And you wouldn't use έναν with είναι anyway -- Είναι έναν λύκο for "it's a wolf" is wrong, because είναι doesn't take a direct object in the accusative case; both "sides" of the verb are in the nominative case.
got it! thanks :D
Why is πράγμα not correct?
It is but you would need: "Είναι ένα πράγμα."
Thank you, yes, I wrote Είναι ένα πράγμα. But he did not except, very strange!