1、泛指某人或某物,但不具体说明是何人或何物;或指代某类中的任何一个。
例如:Be careful! There is a dog lying on the ground.
An English-English dictionary is very necessary for me.
2、用于首次提到的人或物的单数可数名词前。
例如:A little boy wants to play with us.
3、用于时间、速度、价格等含义的名词前,有"每一……"的意思,相当于every。
例如:Take two pills a day and you’ll get better soon.
We go swimming four times a month.
4、用于序数词前,意为"又一,再一"。
例如:Would you like a second cup of tea?
5、用表示具有某种特性的人或物的专有姓名前,相当于a certain ,意为"某一个"。
例如:A Mr. Wang came to see you this morning.
6、用于具体化了的物质名词或抽象名词前,表示"一阵、一场、一种、一次……"或"具体的人或物"。
例如:I would like a coffee and two beers.
The sports meeting was a great success.
7、用于表示日、月、季节、星期等名词前,表示具体的时间。
例如:We had a warm winter last year.