I'm sorry to take this blog as a place of my praticng English.It is kinda awful to do this but I have to.Perhaps you'll love it.
1.at a time 逐一地，一次地，同时
at one time 从前，曾经，一度
It will take more than an hour to finish the job.
指1小时以上2小时以下
It will take more than one hour to finish the job.
指不止1小时，可能要2小时，3小时
2.as a man 从个人观点看
as one man 一致地
I speak as a man, not as a president.
The audience stood as one man and applauded.
3.在相反或相对的食物的应用中，一般用one，不用a(n).
One man's meat is another man's poison.
That's only one way of translating it,but there are other and better ways.
4.a(n)和one在下列句子中可相互替换
in a/one word 简而言之
be of a/one mind 看法一致
at a/one blow 一下子，一击，一举（=at a/one stroke）
a/one hundred percent 十足的
注意：one evening 不等于 on an evening
a friend of mine 不等于 one of my friends
