A grammarian identifies “lā taṭluʿ” as prohibitive lā followed by a jussive imperfect. That analysis is correct, but rhetorical study goes further.
Syntactic question: Why is the verb jussive, and who is its subject?
Rhetorical question: Why tell dawn not to rise when dawn is not an obedient human addressee?
Answer: The expression voices a wish to delay morning, not an enforceable command.
Likewise “lā tabraḥ” can retain its grammar whether a teacher forbids departure or a friend requests someone to stay. Grammar explains construction; rhetoric explains intended force in context.