“Here is a useful book.”当中为什么要用定冠词a,请详细说明.

改错 there are sixty minutes in a hour here is an useful book A dictionary is an useful book.找错 O_____ ,it is a useful book How useful a book it is! This is quite a useful book . 为什么useful前有时用a（a useful book）,有时用an（an useful animal）啊 ( )useful book a/an Here is a book of there are no dogs here还是there is no dogs here,请详细说明为什么, what a useful book it “This is ______ most useful reference book,” a teacher from 英语中的元音字母有哪些?a useful book 为什么用a?