a用在以辅音字母开头，或以读做辅音的元音字母开头的单词前面：a man一个男人a university一所大学a hat一顶帽子a European一个欧洲人a one-way street一条单行马路an用在以元音字母（a，e，i，o，u）开头，或以不发音的h字母开头的单词前面：an apple一个苹果an island一个岛an uncle一位大叔an onion一个洋葱an egg一个鸡蛋an hour一小时an还用在发音以元音开头的单个字母前面：an L－plate一块“实习驾驶”车牌an MP一个国会议员an SOS一个呼救信号an…x‟一个x字母、X形的东西或未知数a／an没有性的变化：a man一个男人a woman一个女人an actor一个男演员an actress一个女演员a table一张桌子a／an不用在以下几种情况下：A 复数名词之前：a／an没有复数形式，所以a dog的复数是dogs，an egg的复数是eggs。
B 不可数名词之前（参见第13节）：C 三餐名称之前，但这些名称前加形容词时除外：We have breakfast at eight．我们8点钟吃早饭。
He gave us a good breakfast．他请我们吃了一顿丰盛的早餐。
为了庆祝某事或特意为某人而举行的宴会之前要用冠词：I was invited to dinner（at their house， in the ordinary way）．他们邀请我吃饭（在他们家吃便饭）。
但是说：I was invited to a dinner given to welcome the new ambassador．我被邀请参加欢迎新任大使的宴会。
A a／an和one（形容词）1 计算时间、测量距离或重量等时，a／an或one可以用于单数的前面：￡l=a／one pound一英镑￡ 1，000，000＝ a／one million pounds 100万英镑（参见第三十六章。
）但是，请注意：The rent is ￡ 100 a week．房租为每星期100英镑。
这个句子中week前面的a不能用one代替。
（参见第2节F。
）在其他类型的陈述句中a／an和one通常不能互换，因为one+名词通常意为 one only／not more than one （只有一个），而a／an则没有这个意思：A shotgun is no good．猎枪不行。
（这种武器不合适。
）One shotgun is no good．一枝猎枪不行。
（我需要两枝或三枝。
）2 one的特殊用法（a） one（形容词／代词）与 another／others对照连用：One（boy）wanted to read，another／others wanted to watch TV．一个（男孩）想看书，另一个／别的男孩们想看电视。
（参见第53节。
）One day he wanted his lunch early， another day he wanted it late．他一天要早点吃午饭，另一天又要晚点吃午饭。
（b） one可以用在 day／week／month／year／summer／winter等词之前，或者用在日期或月份的名称之前，以特指某事发生的时间：One night there was a terrible storm．一天晚上有一场特大的风暴。
One winter the snow fell early．有一年冬天雪下得早。
On e day a telgram arrived．有一天来了一封电报。
（c） one day也可用来表示 at some future date（将来有一天）：One day you‟ll be sorry you treated him so badly．（这里也可用someday。
）终有一天你会因为待他这么不好而后悔的。
（关于one和you，参见第68节。
）B a／an和one（代词）one是可以用来代替a／an的相应的代词形式：-Did you get a ticket？-Yes，I managed to get one．-你搞到票了吗？-是的，我设法搞到了一张。
具有这种用法的one的复数形式是some：-Did you get tickets？-Yes，I managed to get some．-你搞到票了吗？-是的，我设法搞了几张。
1.一(个，件，…)：He had a book under his arm．他手臂下夹着一本书。
He was a boy from a poor home. 他是来自穷苦家庭的孩子。
I saw a girl watering the flowers. 我看见一个女孩在浇花。
Anna had a boy friend named James. 安娜有个男朋友名叫詹姆斯。
He arrived half an hour ago. 他是半小时前到达的。
He is an able man. 他是一个能干的人。
Give him an inch and he will take a mile. 他会得寸进尺。
The book is based on an actual case. 这书是根据一个实际案例写的。
He decided to put an advertisement in the newspapers. 他决定在报上登一份广告。
2.任何…都：A bird has wings. 鸟有翅膀。
A square has four sides. 正方形有四条边。
A dog is an animal. 狗是动物。
3.每(一)：He comes home once a week. 他每周回一次家。
The doctor told him to bathe his eyes twice a day. 医生让他每天洗两次眼睛。
The ducks were force-fed several times a day. 这种鸭子每天强行喂食好几次。
4.某，某一个：A Mr. Black just came to see you. 刚才一个叫布莱克先生的人来看你。
The house next door has been bought by a Mr Jones. 隔壁的房子被一个叫琼斯的先生的人买下了。
【考点说明】1. 用 a 还是用an：一般说来，在辅音或半元音开头的词前用a，而在元音开头的词前用an。
注意：有些以元音字母开头的单词，由于第一个音不是元音而是辅音，其前仍用a而不用an。
2. 单数可数名词若泛指，其前需加 a / an，不要从汉语习惯出发，漏掉此不定冠词，如不能说Her father is famous film poet.(应在is加a)3.不能与指示代词、物主代词、名词所有格等连用，如不说 my a good friend，可改说 a good friend of mine。
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。