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.
why do we use dem Lehrer here?
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"
Go back to the "Dative Case" lesson and read the lightbulb lesson again.
"mit" takes the dative case.
The other translations are incorrect. THAT teacher would be diesem, not dem
How do you know if teacher is plural or singular in this sentence?
if i'm not mistaken the same rule apply in english to some extent, although bypassing gender, in that it's "with whom" and not "with who".
I wrote: I'm eating with the male teacher, but it was wrong.
Why, have you been naughty? You have to eat lunch at some point, but you cant be trusted on your own.
Duolingo keep accepting misspelling without suggesting typo etc. Have any one seen this happen recently? It's happening too often now.