幾時用one,,幾時用a,an??

首先要注意雖然 a/an同one都係determiner(類似adjective 形容詞咁la),
a/an係indefinite articles, 而one係一個數目,

articles的作用是用來顯示一個noun究竟係講一樣general ge野定係某樣指定ge野
indefinite articles是用來表示我地講緊一樣general 的物件.

e.g., Do you have a pen? 我地加'a'因為exactly邊一支筆唔重要, 係筆就ok.
同理, A pig has four legs. A teacher must have patience.都係講緊一d general ge野.

'one'係用黎強調數目, 而唔係refer to一件general的物件.
e.g., I have one pen. 我地想強調我得一支筆.
I have a pen. 是指我有筆.

further examples:
A pig has four legs. 好概括咁講豬有四隻腳.
One pig has four legs. 會出現係小學數學題度...一隻豬有4隻腳, 咁xx隻豬有幾多隻腳?

所以, 主要係睇你用係邊la

有時兩者都用得, 就好似你那兩個例子咁, 兩個意思相近,
不過, 獨立一句 (an old brave lion roars)多數就會用 'a' lor
而如果有一張list數數目的時候, 就用one
即係
I have a brother.
I have one sister, two brothers, three cousins, and five uncles.

不過有d情況, 則不能使用'one'
e.g., He is a doctor. (NOT He is one doctor.)
He is a good person. (NOT He is one good person.) 不過睇電影都聽過d口語咁讀, 但其實唔正統.
Do you have a pet? (NOT Do you have one pet?) '你有無養寵物' vs. '你有無養一隻寵物'

至於,
one hundred同a hundred, 就沒有分別了
不過one hundred比較formal d, a hundred/ a thousand呢d係口語


好似講到好亂咁, 希望你get到

2008-01-08 10:47:15 補充：
a/an 是冠詞, 不是量詞根本上所有countable的名詞都需要冠詞的, 不能說, I am boy. Doctor tells me xxx.除非有其他字取代e.g., my pen, my doctor同埋某些情況下可省略

1. 'One' is used when you want to specify a certain number. For example, I have one dog and two cats.

2. a / an we called it article. an is especially for words with vowels 'aeiou'
For example,
I have a little cat. She is very cute.
This is an orange.

其實可以諗得簡單少少, 就係

- 當你想強調subject/ object既"數量"時, 就用"數字"去表達;

- 如果"數量"對整個statement根本冇大作用, 或者影響唔到個意思的話, 就可以唔用數字去表達, 就咁用"量詞"就可以~~i.e. a, an. 正如第二句咁, 因為要清楚強調只係有"一隻"貓, 但有"兩隻"狗響度同時叫緊, 所以就要用number去表示喇!

希望幫到你~~