非但 is a formal conjunction meaning 'not only' and is almost always followed by '反而' (fǎn'ér, 'but rather') or '而且' (érqiě, 'moreover') to introduce a contrasting or intensifying second clause. Literally, 非 means 'not' and 但 means 'but' or 'only', so together they express negation of limitation — i.e., 'not just... but also...' or 'far from just... in fact...'. It’s commonly used in written Chinese and formal speech, rarely in casual conversation.

This structure emphasizes unexpected contrast or escalation: the first clause sets up an expectation, and the second clause overturns or intensifies it. For example, instead of simply saying someone is smart, 非但 implies that their intelligence goes beyond ordinary expectations — perhaps even surprising or counterintuitive. Because of its literary tone, learners should avoid using it in everyday chats unless imitating formal writing or speeches.