An ELL post (buttonholes -- meaning?) says
I don't think I understand what kind of buttonholes they're talking about.
I found "I don't understand" is commonly used to express the unawareness of something. Google Ngram may verify that
So, what is the difference between "I don't think I understand" and "I don't understand"? Is the latter more formal than the former, therefor no published book uses the former?
Could someone please give a hint? Thanks in advance.
On Google, "I'm not sure I understand" got 17,300,000 hits, "I don't think I understand" got 48,500,000 hits, "I don't understand" got 162,000,000 hits.