need 提问
肯定回答 yes,主语+need.否定回答no,主语+needn't.
must提问
肯定回答 yes,主语+must (have to )否定回答no,主语+needn't（don't have to ）
may提问
肯定回答 yes,主语+can.否定回答no,主语+can't.