英语中“a”和“an”的具体用法？

定冠词（indefinite articles） a/an 的用法。

　　不定冠词最基本的用途就是用来表示“一”这概念，如：

　　① A stitch in time saves nine.

　　② Jason is an internationally known scholar.

　　此外，a/an 还有下列 6 种用途：

　　一常和time、measurement等有关的名词连用，以表示“每—” 的概念，如：

　　③ I teach five days a week.

　　④ This type of vegetables is sold at one dollar a kilo.

　　⑤ My car usually runs sixty kilometres an hour.

　　二常和 hundred, thousand, dozen 以及数目及数量连用，如：

　　⑥ That factory turns out at least a hundred tyres a day.

　　⑦ Noel's monthly salary is a thousand dollars.

　　⑧ I have a number of sponsored students.

　　⑨ Mary has a lot of money.

　　三在“of / at”后面出现，以表示“同一类”这概念，如：

　　⑩ Birds of a feather flock together.

　　11. Please come one at a time.

　　12. Things of a kind come together, so do people of a mind.

　　四常在“rather, quite, many, half, what, such”等字后面出现，形成固定用法，如：

　　13. Tom is rather a fool.

　　14. I think Chinese is quite a useful language.

　　15. Many a student has asked that question.

　　16. The visitor left half an hour ago.

　　17. What a fine day it is!

　　18. How can you say such a thing?

　　五用在“so, as, too, how +形容词”这些结构里，如：

　　19. We have not had so hot a day before.

　　20. Susan is as clever a girl as Anna.

　　21. That is too difficult a book for beginners.

　　22. No one knew how serious a problem it was until later.

　　六出现在许多惯用语中，如：

　　23. Bob always has a bone to pick with others.

　　24. Some students turned a deaf ear to the teacher's advice.

　　25. All must learn a language with an eye to mastering it.

　　26. I hope you will make an effort to attend the meeting punctually.

　　27. Jason has an aversion to being idle.

　　28. The news of Jack's sudden resignation came as a bolt from the blue.

不定冠词a （an）与数词one 同源，是"一个"的意思。a用于辅音音素前，一般读作[e]，而an则用于元音音素前，一般读做[en]。

　1） 表示"一个"，意为one；指某人或某物，意为a certain。例如：

　　 A Mr. Ling is waiting for you. 有位姓凌的先生在等你。

　2） 代表一类人或物。例如：

　　 A knife is a tool for cutting with. 刀是切割的工具。

　　 Mr. Smith is an engineer. 史密斯先生是工程师。

　3） 组成词组或成语，如a little / a few / a lot / a type of / a pile / a great many / many a / as a rule / in a hurry / in a minute / in a word / in a short while / after a while / have a cold / have a try /keep an eye on / all of a sudden等

a和an同样是一个的意思的，但用法不同.an的后面加的名词一定要是元音字母开头,比如:an apple,an egg..

a.e.i.o.u.这些就是元音．所以如果单词前是由这五个元音字母组成的话，那么就要用an．如果不是就反之用a.

楼上的不对
不是元音字母，而是发音
第一个音节发a,e,i,o,u的音的用an
其他用a