1、Some侧重说话人对所述之人或事不明确或一无所知，它只和单数名词连用zhi，其前没有不定冠词。
例如：
There must be some mistake.
一定是出了什么差错。
He's in some kind of trouble.
他遇到了什么麻烦。
She always thinks some guy is going to come along and fix her life.
她总是认为，会有某个男人来到她的身边，把她的生活安排得井井有条。
I'll see you again some time, I'm sure.
我敢肯定，什么时候我们还会再见面的。
Some person called on you when you were out.
你出去时有个人来拜访你。
There must be some reason for what he’s done.
他这么做一定有某种理由。
2、Certain多表示说话人对所述之人或事或知之不详或知而不言，它与单数名词连用时，其前必须加不定冠词。另外，表达此意时，certain还可以同复数名词连用，表示“某些”。
例如：
A certain person telephoned you while you were out.
你出去时有个人给你打过电话。
She managed to arrange for them to be hidden in the house of a certain Father Boduen.
她设法安排他们藏在一个叫博迪恩的神父家里。
It was a certain Dr Davis who performed the operation.
是一位叫戴维斯的医生做的手术。
Certain people might disagree with this.
某些人对这事可能不会赞同。
They refused to release their hostages unless certain conditions were met.
除非某些条件得到满足，否则他们不会释放人质。
For certain reasons I’ll be unable to attend the meeting.
由于某些原因我不能参加此次会议。
For certain personal reasons I shall not be able to attend.
由于某些个人原因，我将不能出席。
Leaflets have been air dropped telling people to leave certain areas.
传单被空投下来，要求人们离开某些地区。
3、在单数普通名词前，a certain和some通常可以互换。在专有名词前通常用a certain，不用some。Some 可以和or…,or other,or another连用，表示未知的对象，而(a)certain没有这样的搭配。
例如：
You owe a certain person a sum of money
你欠某人一笔钱。
《柯林斯高阶英汉双解学习词典》
A certain Smith is waiting at the gate.
有个叫史密斯的人正在门口正等着你。
He used to work at some/a certain place in Guangzhou.
他过去在广州某个地方工作。
She won a competition in some newspaper or other.
她在某报举办的竞赛中得了奖。
Jim Partridge chucked himself off some bridge or other
吉姆·帕特里奇从某座桥上跳了下去。
