怎样变这个主动句为被动语态?

题目

怎样变这个主动句为被动语态?
They don't use the room(他们未使用该房间）,变为被动语态,我觉得应是The room does't used by them,而答案是The room isn't used by them 为什么是isn't 而不是doesn't啊,

答案

被动形式的基本结构是be+动词过去分词,然后各种时态都在be上变化,不会存在助动词doesn't