I don't understand why I am using dem and not den in this case. Nothing has been said about when to use which form.
This is classic german cases ruining german for learners. Certain prepositions (like mit) always take a certain case (in this case dative). You basically just rote learn it.
The other translations are incorrect. THAT teacher would be diesem, not dem
because it is the Dative. In German you ask "Mit WEM esse ich? "Mit dem Lehrer" "Mit DIR".
sentence with accusative would be "I see the teacher" "Ich sehe den Lehrer". Now you ask "WEN sehe ich?" "Den Lehrer" "Ich sehe DICH"
It would be "Ich esse mit den Lehrer" as the plural 'die' goes to 'den' in dative, whereas the masculine 'der' goes to 'dem' in dative.
That's not quite correct.
Ich esse mit dem Lehrer. = I'm eating with the teacher.
Ich esse mit den Lehrern. = I'm eating with the teachers.
Don't see why. You should be right. Probably just duolingo being annoying and acting up, eh?