中为什么不能用“I don’t know”表示“我不知道”？
You got me.
Over my head.
Haven't a clue.
How should I know ? / Who knows ？
In life we are at some stage going to have to use the expression"I don't know", so as variety is the spice of life, using different ways to say "I don’t know" would make your conversation far more interesting and natural.
生活中我们常常会用到“I don't know”（我不知道）这个表达，可总这样说似乎让生活少了点情趣，用其他的表达来表示相同的意思会让你的话更加有趣和自然。
This word is used informally and in spoken language, especially by teenagers who don’t want to answer their parents!
When do you have to submit this essay?
Dunno. I can’t remember.
2. I have no idea/I haven’t a clue/I haven’t the faintest idea
These expressions are used when you have no information and you cannot guess the answer to a question.
What time does the film start?
I have no idea. Why don’t you call the cinema?
Would you by any chance know where St James’s Street is?
I haven’t a clue. Sorry.
Could somebody please explain how this happened?
I haven’t the faintest idea.
3. How should I know?/Don’t ask me/Search me
These expressions are used when you do not know something and you feel annoyed that someone is asking you about it.
Who left this mess on the table?
How should I know? I’ve only just come home.
What time did she say she was catching her train?
Don’t ask me. I’m always the last to know.
Why didn’t he ask you for the keys?
Search me. I’m not a mind reader.
4. Who knows?/It’s anyone’s guess
These are used for saying that you don’t know something because it is impossible for anyone to know it.
Will they ever find the missing aircraft?
Who knows? The search been going on for such a long time now.
How the situation will develop from here is anyone’s guess.
5. Your guess is as good as mine
This is used for saying that you know as little about something as the person who asked you about it.
Do you think we will be able to book the restaurant for Friday?’
Your guess is as good as mine.
6. Not as far as I know
We use this to say that something may be true, but you do not have enough information to know whether it is or not.
Has Clive left the company? I haven’t seen him for ages.
Not as far as I know, but I haven’t seen him recently either.
7. It beats me/You got me/over my head
This is a rather informal expression to say that you do not know or understand something. "It" is often left out.
Why did he do such a stupid thing?
It beats me./You got me./It's over my head.