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? useful前用a还是用an例:____useful book用a还是an? an useful animal?还是a useful animal? an useful book?要说清楚原因,到底是加a还是an,是因为book是辅音还是因为useful是辅音才加a? useful前是加a还是an an ufesul还是a useful It is ( ) useful book.用a还是an?好人一生幸福. There is a(an) useful book and some pens in it如题 用 a 还是 an 为什么useful前有时用a（a useful book）,有时用an（an useful animal）啊 He has _____ useful book.(填an或a) A dictionary is an useful book.找错哪里错了? useful one前用an还是a useful前面用an还是a?为什么? And the book还是a book?What ( ) useful book!And ( )book is popular with students.A.a,a B.an,aC.the,the D.a,the选择题,对的加10分,先加五分! it's（ ）useful book a.quite b.a quite c.quite an d.an quite