为什么They are mending the desk的被动语态不是The desk is ment by them

应该是
The desk is being ment by them.
They are mending the desk是进行时,应该用
进行时的被动

The desk is mending by them

mend好象没被动语态的

不是。要改为；
The desk is being mended by them.

注意：mend是个规则动词，过去分词加-ed.