My mother wants to buy a toy for her. 对 buy a toy for her.进行提问

What does your mother want to do？

My mother wants to buy a toy for her.
我的母亲想要为她买一个玩具.
buy a toy for her
为她买一个玩具
What does your mother want to do？

考察want to do 结构
do也就是buy 对do提问所以借助助动词 does 提问做了什么
所以就是 what does your mother want to do?

My mother wants to buy a toy for her.
我的母亲想要为她买一个玩具
画线部分是WANT 的宾语,所以要提问就得问"想要作什么?"
所以应该问What does your mother want to do？