I don't know VS. I don't understand

[optical]

This is becoming very tiresome. People keep asking me a question in which I want to say "I don't know [the answer]". The person then tries to explain to me the meaning of the question, as if I said "I don't understand." This is what I've gathered so far.

明白 = to understand

知道 = to know

我不明白 = I don't understand

我不知道 = Could be both, I don't know, and I don't understand

So how can I say without any question or doubt, "I don't know" and not "I don't understand?"

[zozzen]

尝试说 “我也不知道 (答案) ” 吧。

我从未遇过这情况, 会不会是你口音太重, 误人家误会你听不明白那句中文呢?

[cdn_in_bj]

我不明白 = I don't understand

I would take this to mean "I don't understand the question"

我不知道 = Could be both, I don't know, and I don't understand

I would take this to mean "I don't know the answer"

However, the fact that you are a foreigner and/or is not 100% fluent may be what's confusing people.

So instead of just replying with "我不明白", you could say "我不明白你（问）的问题" or to be more succinct "不明白,再说一遍"

Similarly, instead of just saying "我不知道", you could include part of the question in the reply. For example, if they're asking for directions to somewhere, you could reply with "我不知道在哪里".

Put yourself in their situation - if you were to ask someone who is not fluent in English a question, and he/she replied with just "I don't know", would you take it to me that he didn't understood your question or that he didn't know the answer?

[studentyoung]

So how can I say without any question or doubt, "I don't know" and not "I don't understand?"

I don’t know something. 我不知道有这回事。

I don’t understand something. 我搞不懂这是怎么一回事。

Thanks!

[Han-tiger]

I couldn’t agree more with cdn_in_bj.

And, from your post, I surmise you may have been put in a situation like this:

a. Actually, you don’t understand the question at all.

b. (Of course you don’t know the answer neither.) what you really want to mean is that you don’t want to answer this question rather than that you don’t know the answer.

In such a situation, the Chinese speaker who is keeping asking you the same question, I think, is kinds of offensive or even rude to you.

You can reply with, “对不起，我不想回答你的问题！”(Sorry, I don’t want to answer your question!)

[muyongshi]

Or if you are just trying to communicate you don't know the answer but understand the question 我不知道答案是什么

[optical]

It looks like zozzen and muyongshi seem to have caught on to what I need. That being, if I say,

我［也］不知道答案［是什么］

I don't think there could be any debate that this means "I don't know the answer" and could not possibly mean, "I don't understand the question" which is exactly what I needed to know. Thanks guys, I'll be sure to memorize this sentence form!