The classroom the door of which is broken will soon be repai

主干是The classroom will soon be repaired.教室会马上被修理的.
中间部分是定语从句部分,其先行词是classroom,the door of which 即the door of the classroom,也就是说这个教室指的是门坏了的那个教室.
把句子分开看看：The classroom the door of which is broken will soon be repaired.
→The classroom will soon be repaired..
The door of the classroom is broken.
门坏了的那间教室很快会被修理的.

the door of which is broken 修饰The classroom （定语从句）
of which is broken 修饰the door
用of which 应为是the door of the class
那间门破了的教室马上就会被修

教室门是破碎的，很快就会被修复。