为什么定语从句 The man who is reading a book 是错的

这个定语从句 The man who is reading a book 之所以是错的,是因为句子的结构不完整,这里主语是he man,定语为who is reading a book,缺少谓语,可以改成这样：The man who is reading a book is my English teacher。正在读书的那个男人是我的英语老师。能看明白么?欢迎交流。