a用于辅音音素前，而an则用于元音音素前。不定冠词是修饰名词的词、是虚词的一种,它本身不能单独使用,也没有词义
1) 表示"一个"，意为one；指某人或某物，意为a certain。
① There is an apple on the table
②A Ms. Lee was here three minutes ago.
2) 代表一类人或物。
Peter is an interpreter.
3) 词组或成语。
after a while / a little bit / a few / a lot /a great many / in a hurry
2) 代表一类人或物。
Peter is an interpreter.
3) 词组或成语。
after a while / a little bit / a few / a lot /a great many / in a hurry
4)和time、measurement等有关的名词连用，以表示“每—” (every)的概念
① I go to English Corner once a week.
② No.7 Bus stops three times an hour.
5)和 hundred, thousand, dozen 以及数目及数量连用：
① Our Company turns out at least a hundred hoses a day.
② Daxi has a dozen of friends in the English Corner.
6)常在“rather, quite, many, half, what, such”等字后面出现，形成固定用法：
①John is rather a fool.
②I think English is quite a useful language.
③Many a student has asked that question.
④The visitor left half an hour ago.
⑤How can you say such a stupid word?
7)用在“so, as, too, how +形容词”：
①We have never had so cold a day before.
②Mary is as slow a girl as Jean.
③This is too difficult an English book for us.
