被动句
1.肯定：Our classroom must be cleaned.
2.否定：Our classroom needn't be cleaned./ Our classroom mustn't be cleaned.(强烈的命令语气.)
3.一般疑问句：Should our classroom be cleaned?疑问句中哪来的Should?提问情态动词的时候可以用should来表示比较委婉的语气，当然你也可以直接用must.