"I once saw an Elephant", isn't the correct answer "Ich habe einmal einen Elefant gesehen", instead of "Ich habe einen Elefanten gesehen" as suggested? "Elefant must be in singular, not in plural, no?
There is no plural here. 'einen Elefanten' is accusative singular (weak declension). http://german.about.com/od/grammar/a/dernouns.htm (last paragraph)
Oh, after all these years I still hate so much the N-Deklination.
Einst sah ich einen Elefanten. What is wrong with that?
Könne ich nur "mal" benutzen?
So is 'Elefant' an adjectival noun? How else would it be used then?
No, it is not -- we don't say der Elefante or ein Elefanter.
It's a weak noun, following the N declension.
Here's one explanation: https://www.duolingo.com/comment/10973844
Here is another one: https://www.duolingo.com/comment/14376869
I ask because it uses an adjective ending here.
Why "Ich sah einmal einen Elefant" is wrong?