"He is a fireman, and not a policeman."
Translation:C'est un pompier et non un policier.
I did that too.
Il est pompier. .... He is a firefighter. Seems obvious.
But, DL wants: C'est un pompier et non un policier.
I think either might be correct, but with the 'un' before "pompier" you have to use "C'est", and that doesn't translate back to English as "He is".
So, it's weird.
C'est can mean both "it is" and "he is", depending on context.
I'm wondering if you have to use "c'est" here because policier is preceded by un and having un before pompier gives the sentence a kind of symmetry? And you can't say "il est un pompier" so you have to say "c'est un pompier".
I'm just guessing though.