Translation:The teacher is good.
Why not "The good teacher."?
It'll be معلم جيد
and what would be then "A good teacher."? ;)
I think it actually is just that Duolingo should allow that answer, report it.
You think wrong.
It's the same, just like "one good teacher".
"The good teacher" would rather be المعلم الجيد , I believe
