哪些u开头的单词中前面加a,哪些u开头的单词中前面加an

哪些u开头的单词中前面加a,哪些u开头的单词中前面加an

词首的 U 发【j】音时,前面用 a ,eg; a university,a used car,a usual day 等.
词首的 U 不 发【j】音时,前面用an an uncomfortable chair,an ugly child 等.