一个人的英语是：a person，或者a single person。注意不能用集体名词people。如果表示一个人的状态，可以说：alone，或者on one's own。

a person/a single person

What can you do with a person like that？
碰上这么一个人，你有什么办法?

Mary was the first person to think of the idea.
玛丽是第一个想到这个主意的人。

The age of a person is usually reckoned from his or her date of birth.
一个人的年龄一般是从他的或她的出生日期算起。

She is an attractive person.
她是个吸引男人的人。

He was a person out of the ordinary.
他是一个不同凡响的人物。

Apart from my host, I didn't know a single person there
除了主人外，那里的人我一个也不认识。

I now have to face the rest of my life as a single person
我现在不得不孤单一人度过余生。

alone

I don't like going out alone at night.
我不喜欢夜晚单独外出。

He lives alone.
他独居独处。

Finally the two of us were alone together .
最后只有我们两人在一起。

She was sitting all alone in the hall.
她一个人坐在大厅里。