dull flame
Hello, could you please help me explain, why we keep subject in Dative Case here, not transform back to Nominative and apply passive?
In English I feel it would be: The students were helped by the teacher, not, To the students were helped by the teacher
primal wing
1. English has no dative
2. 'To help' does not even use the English pseudo-dative construction 'to the students', these are just simply different verb constructions in the two languages; however, English verbs that do use this pseudo-dative do still flip into subjective case - He gave the ball to the students :: The students were given the ball
3. There really is no 'why', dative things simply cannot become a subject in German. Take the sentence 'Mir wurde gesagt', here you may think 'mir' is the subject, but the real subject is a hidden 'es' - Mir wurde (es) gesagt.
I think it's important to stress that last point: 'Den Schülern' is not the subject. Things not in Nominativ cannot be a subject.
primal wing
I wouldn't recommend using this quite yet as a learner, but as a circumvention of this dativ-passiv, some people use what's called the Bekommen-Passiv; more info here: https://deutschegrammatik20.de/passiv/passiversatz-bekommen-gehoeren/