不定冠词a/an的用法：
不定冠词有两个，a 和 an。a用在辅音开头的'单词前面;an用于以元音开头的单词前。如：a book, an apple。具体用法如下：
1)表示“一个”、“某一个”、“ 每一”、“再一，又一”、或“同一，相同”等
Eg：
Rome is not built in a day.
A Mr. Smith is waiting for you at the school gate.
He usually works eight hours a day.
You can try it a second time if you fail.
They are of an age./ Birds of a feather flocks together.
2)泛指某一类人或物，意为“任何一个，任一”
Eg:
A square has four sides.
3)当物质名词具体化表示“一(类、阵、份)”等;抽象名词具体化为“……的人或者事”时
Eg:
Green tea is a wonderful tea.
We don’t have much rain here, but last night we had a heavy rain.
Reading is a pleasure for me.
The meeting is a great success.
4)表示季节、月份、日期、三餐的名词，或表示世上独一无二的物体名词前有形容词修饰时：
Eg:
We have a every cold winter last year.
This happened on a rain Sunday towards the end of May.
We hope we can see a full moon tonight.
5)源于动词的名词与take/have 连用时：
Take a look; take a walk; have a break; have a visit to…