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)