a / an表示每一：
three times a day
A Mr. Li is waiting for you.
a / an 区分
a useful book / a one-eyed camel / a European village / a uniform / a university
a usual thing
an unusual thing / an hour / an umbrella / an honest boy / an English book
an apple / an eight-year-old boy / an unhappy boy
26个英文字母单独使用时，泛指前an的字母是：Mr. Li has one fox.
例如：There is an o in the word worry.