Hi Christophe how are you? 1) "I don't know" is a bit stronger. You are positive that you don't know something it is more of definite answer, then "I'm not sure." 2) "I'm not sure" is less strong because you are unsure of the a certain thing or possible situation. It could be the right thing to do but maybe you are only like 50% sure. Let me know if that makes sense!

Okay, I have received usefull answers here but I still have a doubt. I noticed many times that english-speaking people use the sentence "I'm not sure" where we french-speaking people would just say "je ne sais pas" (which is the direct translation of "I don't know"). So, it seems to me there's something more subtle than a sheer level of uncertainty. Someone once told me it's also a question of politness. "I don't know" can be perceived in some context as a rude reply. Is it so?

I agree with the others. “I don’t know” implies that you have no knowledge of the subject whereas “I’m not sure” implies you have some knowledge, but it may be incorrect. Example: “Hey, who is that woman talking with Claire?” Answer: “ I don’t know, I’ve never seen her before.” Or “I’m not sure, but I think it’s Claire’s sister. I heard she was in town visiting.”

The difference is that I don't know is when a person knows nothing or less about something but does not want to share it due to absent or insufficient information while I'm not sure is when someone knows about something but does not know the full details but would like to share the little that he or she knows. I don't know is used when one is approached with a topic or question that they have never heard and know nothing about, or maybe heard but do not know any facts about it. I'm not sure is used when one knows about something and is speaking under corrections, meaning that full details are not fully known to him/her.