有人 / 没人【yǒu rén / méi rén】

有人 + verb means "someone" or "some people" did it. When you don't know who it was, 有人 introduces the person, and the verb follows.

"Nobody": 没人

没人 or 没有人 + verb is "nobody", "no one".

"Is anyone…?"

In a question, 有人 is "anyone".

有人…有人…

Repeated, it splits a group: "some … and others …".

Not 一个人 or a bare 人

"Someone" isn't 一个人 at the head of a sentence, and 人 on its own can't start one with this meaning.

• × 一个人在敲门 → 有人在敲门
• × 人找你 → 有人找你

有的人 also means "some people" and can be the subject directly: 有的人喜欢, 有的人不喜欢.