an useful book还是a usefull book?

an useful book还是a usefull book? ( )useful book a/an 是an useful book还是a useful book 加a 或者an 主要看什么 是a useful book,还是an useful book.为什么?不是所有元音开头的单词前面都加an吗? A dictionary is an useful book.找错 He has _____ useful book.(填an或a) what a useful book it 为什么useful前有时用a（a useful book）,有时用an（an useful animal）啊 是an English book还是a English book There is a interesting book.还是an interesting book? 是an English book还是a English book? it's（ ）useful book a.quite b.a quite c.quite an d.an quite