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".