为什么用an exam而不用a exam?

用have an exam,take an exam,examine各造一句!） 用fail an exam 造句 take an exam 英语中的hour前面为什么用an,而不用a? 英语作文After an exam hold an exam/give an exam/take an exam/have an exam在用法和意思上有什 fit an exam的中文 have an exam 的造句 Well-done on an Exam have an exam的造句 a exam for/about take a big exam