谓语动词bought　（买）是buy　的过去,实义动词,变一般疑问时,用助动词do /does 提前.
但我们看到　bought 是过去式,表过去的事情,助动词do/does 变过去式　did
Did Jenny buy a star yesterday?
助动词提前,后接谓语动词用原形.