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Don't get off before the train has stopped.
Literal
Train [subject-が] stop [while-not-yet-ないうちに] get-off must-not.
~ないうちに ('before X happens, while X hasn't yet happened') is a subtle temporal pattern — marks the window before some change. Literally 'during the time when X has not yet,' it's the positive framing of a negative — 'before it stops.' 降りる ('to get off') + ~てはいけない as prohibition.