why is it 'da insegnante', and not 'come un insegnante'?
Because come un insegnante means 'like a teacher', and in this sentence 'like a teacher' would make no sense.
'As' (someone/something) translates as da.
grazie mille. Non ho visto niente la parola 'da' usato in questo modo
You are welcome.
You can still use come with the same meaning as da , but in either case you must omit the article:
da insegnante, .... = as a teacher, ...
come insegnante, ... = as a teacher, ...
come un insegnante, ... = like a teacher,...
Duo is insisting on "As A/ONE teacher..." in this sentence, but what is there in the words to definitively preclude "As THE teacher..."?
l'insegnante? According to Deepl that would change the meaning though.
What is wrong with: ...only I can agree with you?
It changes the meaning in English. "I can only agree with you" -- I can't argue the other side, I agree. "Only I can agree with you" -- I am the only person that agrees with you, everyone else does not.