a 是for consonant such as(b, c, d, f)
an is use for a, e, i, o, u which is normally known for vowels.
but
there are also some special case, such as silent h-(hour)
Although h is a consonant but it sounds like ao so silent h- is use for an
你以为没有其它special case了吗?漏!
还有多一个
你现在读一下university
那个u是不是sounds like yu-?
so这一种情况呢,我们应该使用a
因为yu的那个y是consonant
希望可以帮助到你,望采纳,谢谢(●'◡'●)