英语中为什么不能用“I don’t know”表示“我不知道”？

Beats me.
难倒我了，我还真不知道。

You got me.
你问倒我了，不晓得。

Over my head.
超出我的范围了，不知道。

Haven’t a clue.
完全没头绪，不知道。

How should I know ? / Who knows ？
我咋知道？谁知道呢？

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。

1. Dunno

This word is used informally and in spoken language, especially by teenagers who don’t want to answer their parents!
这个词为非正式用语，一般在口语中使用，孩子们不想回答父母的问题时会经常用到这个词。

When do you have to submit this essay?
你什么时候要交论文？

Dunno. I can’t remember.
不知道，不记得了。

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.

这些表达通常用在你没有任何关于这个问题的信息，无法猜到答案的时候。

What time does the film start?
电影什么时候开始？

I have no idea. Why don’t you call the cinema?
不清楚，你为什么不打电话问电影院呢？

Would you by any chance know where St James’s Street is?
你知道圣詹姆斯大街在哪儿吗？

I haven’t a clue. Sorry.
不好意思，不太清楚。

Could somebody please explain how this happened?
有没有人能解释一下这是怎么回事？

I haven’t the faintest idea.
我也不知道。

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.
当你不知道问题的答案，又很反感别人问你时，可以这么说。

Who left this mess on the table?
谁把桌子弄得一团乱？

How should I know? I’ve only just come home.
我怎么知道，我才刚到家。

What time did she say she was catching her train?
她什么时候说她在赶火车来着？

Don’t ask me. I’m always the last to know.
别问我，我总是最后一个知道的。

Why didn’t he ask you for the keys?
为什么他不问你要钥匙呢？

Search me. I’m not a mind reader.
不知道，我又不会读心术。

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.
当你不知道问题的答案，而且其他人也不可能知道的时候，就可以用这两句。

Will they ever find the missing aircraft?
他们会找到失踪的飞机吗？

Who knows? The search been going on for such a long time now.
谁知道呢？搜寻工作已经进行了这么长时间了。

How the situation will develop from here is anyone’s guess.
谁都不知道事态会如何发展。

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.
如果你和提问题的人一样不知道问题的答案，你就可以这样说。

Do you think we will be able to book the restaurant for Friday?’
你觉得我们能在周五预定到这家餐厅吗？

Your guess is as good as mine.
我和你一样不知道。

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.
当你认为某件事可能是真的但又不确定的情况下，可以用这个表达。

Has Clive left the company? I haven’t seen him for ages.
Clive离开公司了吗？我好久都没见到他了。

Not as far as I know, but I haven’t seen him recently either.
可能吧，我也很久没见他了。

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.
这是一种表达你不知道或不懂的非正式用语。“it”可省去。

Why did he do such a stupid thing?
他怎么会做这种蠢事？

It beats me./You got me./It’s over my head.
不知道啊！