在这篇短文里，我想谈谈不定冠词a和数词one有哪些相同与不同之处。而这也正是中国学生常会遇到的一个问题，不是吗？
让我们先看看这两个词有哪些不同吧。a和one虽然都可以译作“一”，但它们却有不少不同之处。
首先，在下列情况下一般都用a而不用one：
1. 表事物的类别而不表事物的数目的可数名词之前用a，这种a可以说相当于any one，如：
(1)A cat is a domestic animal.
(2)A burnt child dreads the fire.
(3)If you want to kill a tortoise, wait till he puts out his head.
2. 当不强调“一”的数目概念时用a，如：
(4)Wait a moment.
(5)He emptied his glass at a draught.
(6)Not a word was said. （not之后常用a）
3. 在不可数名词之前表“一部分”含义时用a，如：
(7)It's a pity that you missed the train.
(8)He has a good knowledge of English.
此外，还有一些含有不可数名词的介词短语也都用a，如in a hurry, in a panic, at a loss, at a gallop等。
4. 表“每一”的概念时用a。这种a或an乃由介词on变来，其意义相当于per,如：
(9)It costs two dollars a pound.
(10)He goes to the city only a few times a year.
5. 在half, such, many, what（感叹词）， quite, rather等词之后一般皆用a，如：
(11)Half a loaf is better than none.
(12)Such a thing has never happened before in this city.
(13)There's many a slip twixt the cup and the lip.
(14)What a day!
(15)It was quite a party.
(16)He's rather a fool.
6. 在a few, a great many等表数目的固定词组中皆用a，如：
(17)Only a few of us went.
(18)The pear tree in the yard bore a great many pears. （在非正式英语中亦可用a good many或a pretty many）
另外在a full six hours, a long five miles, an estimated 3,000 people, a number of books等常用说法中也都用a。同时请注意在表度量的分数中亦用a，如：
(19)I bought half a pound of tea and a quarter of a pound of coffee.
7. 在“have+a+look等动作名词”结构中常用a，如：
(20)Have a look at this photo.
(21)Will you have a drink?
(22)I had a swim in the river yesterday.
在这种结构中，have还可后接a think, a smoke, a bite, a chew, a weep, a row等。
8. 还有很多习语用a而不用one，这里只举几例： a kind of, to a degree, a sea of faces, a mountain of work, once upon a time, to a man, of a morning， of a mind (a=the same)。
现在再让我们看看常用one而不用a的情况：
1. 强调“一”的数目概念而且有与two, three等其他数目对比的含义时，如：
(23)You can have one apple.
(24)It was one coffee we ordered, not two. （one在此与two对比，但a有时亦可与two, three等对比，如She has two daughters and a son.）
但在某些格言中须用a，如：
(25)A stitch in time saves nine.
(26)A bird in the hand is worth two in the bush.
2. 表“只有一个不多于一”的概念时，如：
(27)There's only one way to do it. （常与only连用）
(28)This is the one thing we can feel certain about. （与定冠词the连用）
(29)It was her one great sorrow. （与物主代词连用）
(30)When she was pregnant again, she decided to give it up so as to follow the Party's call to limit families to one child.
3. 与other, another连用或对比时，如：
(31)What I say goes in at one ear and out at the other.
(32)He had one good coat and two others that were dirty.
(33)One man's meat is another man's poison.
(34)I've managed to save quite a bit one way and another.
4. 有些固定说法须用one, 如as one man; with one voice; one man, one vote; one last time （chocolate, etc.）等。
上面所谈到的a与one的各自用法似乎界限分明，不易混淆。但事实上并非完全如此，现在我们举例说明如下：
1. a与one皆可表“某一”的概念，但二者的使用范围不同。a的使用范围很广，如：
(35)A man called this afternoon.
(36)There's a lion escaped from the zoo.
(37)He works at a factory in Shanghai.
这样的a也可以用于人名，如：
(38)Mr. Smith, there's a Mr. Brown to see you. （这里说话人用a表示他只知Mr. Brown其名，而不知他是什么人。英语中亦可用one于人名之前，如one Brown，但one在此不是限定词，而是一支撑词）
one表“某一”的范围则要小的多，一般常与day, night, morning, afternoon, evening等词连用，如：
(39)One day, many years later, I found out what had really happened.
(40)I'd like to go to India one day.
(41)One evening I arrived late.
(42)A French examination was announced for one Friday late in August.
有时这种表“某一”的one也用于其他名词之前，如：
(43)The work was well done in one way. （in one way=in a sort of way）
(44)At one point, he thought he saw a bulldozer about to crush him.
(45)He would perhaps visit one duchess in Mayfair, concerning the George Richard Memorial.
2. a与one皆常可用在hundred, thousand, million等数词之前，如a hundred和one hundred, 但在日常用语中多用a。用a时没有与two, three等对比的含义，用one时则有此含义。
此外，还应注意以下几点：
1. 在较长数字中，a hundred等只可用在其开头，如a hundred and thirty-two。但hundred等数词如处在较长数字的其他位置，其前则须用one, 如two thousand，one hundred and thirty-two。
2. 数词thousand之后如用and, thousand之前可用a, 如a thousand and thirty-two; thousand之后如为百位数，则其前须用one，如one thousand, three hundred and thirty-two。
3. 有些习语必须用a, 如：（not)one in a thousand, a thousand thanks, a hundred（或thousand）and one（意谓“许多”）等。
4. 阿拉伯数码中的1一律读作one，如1，100须读作one thousand one hundred。同样，125 BC须读作one hundred (and)twenty-five BC, ￡1.25须读作one pound twenty-five。
5. 可以说a mile, a foot, a pound, a dollar, 但常须说one mile two hundred yards, one foot ten (inches), one pound fifty, one dollar twenty-five。但既可说one hour twenty minutes, 也可说an hour and twenty minutes。
英语里有不少说法既可用a又可用one，但二者的意义却不相同，现略举数例比较如下：
(46)at a time 一次，每次
at one time 曾经；一度
(47)as a man 作为人（指其性格或品格）
as one man 像一个人一样（意谓团结一致）
(48)more than a year 一年多一点
more than one year 一年以上（即不止一年）
(49)for a moment 一会儿（a是一般用法）
for one moment 一会儿（用one语气较强）
英语里也有一些说法用a或one均可，二者的意义无甚区别，如：
(50)in a word
in one word
(51)of a mind
of one mind
(52)a hundred per cent
one hundred per cent
(53)at a blow
at one blow
下面各对说法中的a与one不可互换，但也都同义：
(54)one evening
on an evening
(55)one or two days
a day or two
(56)one of my friends
a friend of mine
最后，必须指出，这里我们只对作为限定词的a与one进行比较。关于one用作非限定词（如后无名词的数词one以及代词或支撑词one），就都不涉及了。
