1. 泛指某类人或物（或其中的任何一个）、初次谈到或见到的人或物。这个用法是不定冠词考查的重点。
?—I knocked over my coffee cup. It went right over the keyboard.
—You shouldn’t put drinks near a computer.
2. 表示“每一”，相当于every/per。
?I earn 10 dollars an hour as a supermarket cashier on Saturdays.
3. 表示“一个”，相当于one。
?My sister sometimes has only an apple for dinner because she wants to be thin.
4. 用于专有名词前表示“某一个……”，相当于a certain；或属于某人的物品。
?—Hello, could I speak to Mr. Smith?
—Sorry, wrong number. There isn’t a Mr. Smith here.
5. 用于序数词前表示“再一，又一”。
?She didn’t like the color of the two bags and asked for a third one.
6. 用于动词转化的名词前，表示一次动作。
?Let’s go and have a look.
