I bought this radio for my aunt in a shop in Shanghai改为被动语态,

主动语态是主语是谓语的动作方,被动语态则相反,主语是谓语的被动作方.
这句话的结构很简单,I是主语,bought是谓语,the radio 是宾语.for my aunt,in a shop in Shanghai这些可以当作修饰,暂时无视.
反过来就是the radio当主语,was bought 当谓语,I当宾语,则主格变宾格.
那么就可以写出 The radio was bought by me.
然后可以将for my aunt,in a shop in Shanghai添入即可.
整句话就是 The radio was bought for my aunt by me in a shop in Shanghai.
就考试而言,这样已经正确了.