Could:肯定：Yes,you can 否定：No,you can't/mustn't(mustn't语气稍强烈）
Must:肯定：Yes,you must/have to 否定：No,you needn't/don't have to(一定不用mustn't)
May:肯定：同could（也和can一样,只是提问时三者语气委婉程度不同）
Should：肯定：Yes,you should/must/have to (后二者语气强硬,有命令意味） 否定：No,you shouldn't/can't/mustn't（后二者有禁止之意）