This is the 212th most frequent Persian word.
شدهاند
"Have become" or "have been" (plural).
The word 'شدهاند' is used to indicate that the subjects (they) have taken on a new state or role, being 'good teachers'.
آنها معلمان خوبی شدهاند.
They have become good teachers.
Here, 'شدهاند' is used in the passive voice to state an action that has been performed on the subjects (books), which is 'being forgotten'.
کتابها فراموش شدهاند.
The books have been forgotten.
In this sentence, 'شدهاند' is part of a question in passive voice, asking if the subject (new rules) has undergone the action 'being accepted'.