Used in rhetorical questions to emphasize disbelief or surprise; often used in the patterns 难道...吗 nán​dào... ma or 难道...不成 nán​dào... bù​chéng, meaning "could it be that..." or "is it possible that..."

Used at the end of a sentence to turn it into a yes-or-no question or to express doubt or a rhetorical question.