I'm not sure.

What's her game?

I have no idea.

Do you know where my pen is?

I have no idea. /I have no clue. /No idea.我一点都不知道

It's all Greek to me. /I didn't know that.

John got married last week.

I didn't know that. 我怎么不知道/That's news to me.

I wasn't aware of that. /I don't know.

Do you know where she's from?

I don't know. /I have no idea.

Do you know the way to my house?

I don't know for sure. “确切地”

I don't know for certain.我不太清楚

I'm not absolutely sure.

No one knows for sure. 谁也不知道确切的情况

Is he married? --How should I know? 我怎么会知道.？/谁能知道？

What is the meaning of life?什么是人生?

Who knows? --Nobody knows.

No way of knowing.

There's no way of knowing.

It's impossible to find out.