The lady who are making the bed is her mother中的错误处是哪儿,错误原因 是

楼上改正都没有问题,但是解释不够清楚.
主句是：The lady is her mother.
而从句who are making the bed是修饰主句主语the lady的定语,称为定语从句.
定语从句的关系代词who等同于先行词the lady,在该定语从句中充当主语.
既然 who = the lady,所以定语从句的be动词当然用第三人称单数形式即is,而不能使用are.
再如：I,who am a student,is studying hard for my future.
其中who = I,所以定语从句中的be动词用am,而不是用is或are.