one 的用法

one 表示“一个”，可作代词或限定词。作代词时，指代单数可数名词，表示人或其他东西；作限定词时，作定语修饰单数可数名词。

one 作代词时在句中的作用如下：
a) 表示一个人或事物:
It's a present for one of my children. 这是送给我的一个孩子的礼物。
There were three coaches and one was black. 有三名教练， 一个是黑人。

b) 用于代替刚提过的可数的事物:
There are many magazines there. I'll get one for you. 那里有很多杂志，我给你拿一本。
There were three pens and one was black. 有三支笔，其中一只是黑色的。

c) 泛指一般的人或指本人:
One could see that he was very upset. 我们可以看出他很难过。
One gets the impression that they disapprove. 本人的印象是， 他们不赞成。

d) 用于辩别所谈的人或事:
Our house is the one next to the school. 我家的房子就是学校旁边的那座
Students who do well in examinations are the ones who ask questions in class. 考试成绩好的都是上课爱提问的学生。

one 也可作限定词，在句中作定语:
There's one thing that I am sure of. 有一点我是很确定的。
There's only one piece of cake left. 只剩下一块饼了。
One afternoon lily went out with Adam. 一天下午莉莉和亚当一块儿出去了。
One day you will regret that you've done so many wrong things. 有一天你会为自己做了那么多错事而感到后悔。