A or an is the indefinite article. It is used at the beginning of noun groups which refer to only one person or thing. The form an is used in front of words that begin with vowel sounds.
a 或 an 为不定冠词，用于表示单个事物或人的名词词组之前。an 用于以元音开始的单词之前。
1DET （指初次提及或非特指的人或物）一（个） You use a or an when you are referring to someone or something for the first time or when people may not know which particular person or thing you are talking about. （指初次提及或非特指的人或物）一（个）
A waiter entered with a tray.
He started eatinganapple.
Today you've got a new teacher taking you.
I managea hotel.
2DET （一类人或事物中非特指的）任何一（个） You use a or an when you are referring to any person or thing of a particular type and do not want to be specific. （一类人或事物中非特指的）任何一（个）
...expensive make-up that we saw being advertised by a beautiful model.
I suggest you leave it to an expert.
Bring a sleeping bag.
I was waiting for a bus.
3DET （用于前加形容词或后加修饰语的不可数名词之前）一（种） You use a or an in front of an uncount noun when that noun follows an adjective, or when the noun is followed by words that describe it more fully. （用于前加形容词或后加修饰语的不可数名词之前）一（种）
There wasa terrible sadness in her eyes.
He did havea real knowledge of the country.
Baseball movies have gainedanappreciation that far outstrips those dealing with any other sport.
4DET （用于物质名词前）一（类），一（种） You use a or an in front of a mass noun when you want to refer to a single type or make of something. （用于物质名词前）一（类），一（种）
Bollinger 'RD' is a rare, highly prized wine.
5DET （用于表示数量的词组中）许多/少许/一点 You use a in quantifiers such as a lot ,a little, and a bit. （用于表示数量的词组中）许多/少许/一点
I spend a lot on expensive jewelry and clothing.
I've come looking for a bit of advice.
6DET （一类人或事物中典型的）一（个） You use a or an to refer to someone or something as a typical member of a group, class, or type. （一类人或事物中典型的）一（个）
Some parents believe a boy must learn to stand up and fight like a man.
...the operationa patient has had.
7DET （用于日子、月份或节日的名称之前以表示特例）一个 You use a or an in front of the names of days, months, or festivals when you are referring to one particular instance of that day, month, or festival. （用于日子、月份或节日的名称之前以表示特例）一个
The interview took place on a Fridayafternoon.
It wasa Christmas when shoppers passed by expensive silks in favor of more practical gifts.
8DET （从事某一职业的）一（位），一（名） You use a or an when you are saying what someone is or what job they have. （从事某一职业的）一（位），一（名）
I explained that I wasanartist.
He was now a teacheranda respectable member of the community.
9DET [DET n-proper]（用于不为说话人所知的首次提到的人名前）某一（个） You use a or an in front of the names of people as a way of indicating that you do not know them or anything about them and you are saying their name for the first time. （用于不为说话人所知的首次提到的人名前）某一（个）
The full address on a stick-on label was that of a Mrs P. R. Slater of Peterborough.
不干胶标签上的完整地址写的是彼得伯勒的一位P. R. 斯莱特太太。
A Dr Matthew Owens was reported missing while on an expedition to north-eastern Turkey.
10DET [DET n-proper]（用于人名前，表示与该人有相同特性或品质）类似…的一个（或一位） You use a or an in front of the names of people when you want to refer to someone else who has the same qualities or character as the person named. （用于人名前，表示与该人有相同特性或品质）类似…的一个（或一位）
When I listen to her play I can heara new Nigel Kennedy.
11DET [DET n-proper]（用于姓氏前，表示某人属于某姓氏家族）一位姓…的人 You use a or an in front of a surname when you want to refer to someone who belongs to the family with that surname. （用于姓氏前，表示某人属于某姓氏家族）一位姓…的人
As faras I can recall, Patti wasa Smith.
12DET （用于艺术家名字前）一幅…的画，一件…的雕塑作品 You use a or an in front of the names of artists to refer to one individual painting or sculpture created by them. （用于艺术家名字前）一幅…的画，一件…的雕塑作品
Most people have very little difficulty in seeing why a Van Gogh is a work of genius.
13DET （代替数字one使用，尤用于hundred，hour和metre等表示度量的单词之前以及half, quarter和third等分数之前） You use a or an instead of the number 'one', especially with words of measurement such as 'hundred', 'hour', and 'metre', and with fractions such as 'half', 'quarter', and 'third'. （代替数字one使用，尤用于hundred，hour和metre等表示度量的单词之前以及half, quarter和third等分数之前）
...more thana thousandacres of land.
...a quarter of an hour.
The skirts were shortened an inch or two.
14DET （用于表示比率的短语中）每一 You use a or an in expressions such as eight hours a day to express a rate or ratio. （用于表示比率的短语中）每一
Prices startat £13.95 a metre for printed cotton.
The helicopter can zip alongatabout 150 kilometres an hour.
1N-VAR 英语字母表的第1个字母A is the first letter of the English alphabet. 英语字母表的第1个字母
2N-VAR C大调音阶中的第六音；A音 In music, A is the sixth note in the scale of C major. C大调音阶中的第六音；A音
3N-VAR （作业、考试等）甲等，优等 If you get an A as a mark for a piece of work or in an exam, your work is extremely good. （作业、考试等）甲等，优等
4（用于首字母为 a 的一些单词的缩写，如 acceleration，ampère和answer）A or a is used as an abbreviation for words beginning with a, such as 'acceleration', 'ampère', or 'answer'. （用于首字母为 a 的一些单词的缩写，如 acceleration，ampère和answer）
5PHRASE 从一处到另一处；从甲地到乙地 People talk about getting from A to B when they are referring generally to journeys they need to make, without saying where the journeys will take them. 从一处到另一处；从甲地到乙地
Cars are for getting people from A to B in maximum safety.