1.不定冠词a 和an的用法区别：a 用在单词发辅音开首的单词前，an用在单词发元音开首的单词前。如：

　　a book, a desk, a pen, a doctor, a house

　　an enemy, an accident, an animal, an egg

　　但注意：an hour, a useful tool

　　2.泛指某人或某事物

　　An old man is sitting on the chair.

　　3.表示one 或every，但概念上没有它们那样强烈。当表示one的意思时，一般不用two, three等相对比。如：

　　Every elephant has a long nose.

　　Take this medicine three times a day.

　　4.用于某些固定词组中。

　　a few, a bit, a lot of, in a hurry, at a time, all of a sudden, many a, a little, a couple of, as a matter of fact, in a word, have a good time, do sb. a favor, pay a visit to, in a way, have a word with, make a living, take a pride in, take a walk, a great deal of, catch a cold,

　　5.用在某些抽象名词前，表示“一种，一类，一场，一阵”等

　　She was caught in a heavy rain.

　　It was a wonderful tea.

　　That is a great disappointment.