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

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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.

  • 1
    The second is more definite. Similarly "I don't think I will attend" and "I will not attend." – Weather Vane Mar 20 '20 at 14:37
  • I'd be prepared to bet money that I'm not sure I understand would occur more often than I don't think I understand in "formal" contexts. Perhaps If they've digitised and indexed Hansard (full transcripts of Parliamentary debates in the UK), this could be easily proved by a quick text search. But neither of those highlighted "hedges" should necessarily be interpreted literally - often they simply reflect polite, deferential hesitancy / circumlocution in formal contexts. – FumbleFingers Mar 20 '20 at 14:47
  • @FumbleFingersReinstateMonica Thank you. "Ngrams not found: I don't think I understand, I'm not sure I understand" – WXJ96163 Mar 20 '20 at 14:54
  • Things like this are nowhere near common enough to chart using NGrams. But if there was an electronically-searchable copy of Hansard, I'm sure I'd be proved right. The king might quite naturally say I don't think you understand me to a slow-witted courtier, but even the slowest-witted courtier would probably know enough to switch to the more deferential I'm not sure I understand you, sire when addressing his sovereign. Being unsure is inherently more "servile / sycophantic" than [not] thinking something. – FumbleFingers Mar 20 '20 at 15:06

In this context, the use of the word "think" is generally used to soften the statement.

"I don't understand" and "I don't think I understand" mean basically the same thing, but the first one is more blunt, and in some cases could be interpreted as confrontational or impolite (with an implication of "I don't understand because you failed to explain it properly", for example), while the second form makes a point of focusing the statement on the speaker themselves and their own thoughts, making it seem more about them than about the other person ("I don't understand, but that's probably my fault").

"I don't think I understand" can also be used to express some amount of confusion, instead of just lack of understanding. It can have the sense of "I thought I understood, but then something else made me think my understanding was actually wrong".

  • Thanks for your comprehensive explanation. Could "I'm not sure I understand" also be used to soften the statement? – WXJ96163 Mar 20 '20 at 22:43
  • Yes, although "I don't think" suggests that you're pretty sure your understanding is not right, while "I'm not sure" suggests that you think your understanding may be right, or it may not. Both can serve to soften the statement, however. – Foogod Mar 20 '20 at 22:45
  • Thank you, that's very kind of you. And then, why does Google Ngram include those expressions at all? Is it because that is informal? – WXJ96163 Mar 20 '20 at 22:48
  • Actually, that surprised me too, but I think I figured out why: Ngrams automatically expands contractions ("I don't understand" --> "I do not understand"). For "I don't think I understand", it converts it to "I do not think I understand", but Ngrams only tracks phrases of five words or less, and the expanded phrase is six words, so it has no data for it (you can see this if you put in "I do not think I understand" yourself, it will tell you it's too many words). If you leave out the first "I" (search for "don't think I understand") you will see some results. – Foogod Mar 20 '20 at 22:53
  • I am pretty sure you are right. I searched them in books.google.com, everything is good. – WXJ96163 Mar 20 '20 at 23:04

"I think" is used to mean you believe something may be true, but you are not certain.

Paris is the capital of France

Is a statement that you are certain of.

I think Budapest is the capital of Hungary

Means that you believe Budapest to be the capital of Hungary but you are not certain. "I don't think..." is just the negative of it.

I don't think Bratislava is the capital of Hungary

In your specific case:

I don't understand X

means that you don't have any understanding of X. You are certain that you don't understand it.

I don't think I understand X

means that you are not sure if you understand X or not. For example you may have some understanding of X but you suspect that it may be wrong.

As @Foogodp points out, "I think..." can also be used as a rhetorical device to soften a statement or make it more polite. For example saying "You are wrong." can be seen as harsh and confrontational, whereas "I think you may be wrong" is less so. The meaning is essentially the same. In the second version you are expressing less certainty (whether you actually have less certainty or not) which makes the statement less harsh.

  • Thanks for your answer. But it's "you". Why are "you" not sure whether "yourself" understand something? – WXJ96163 Mar 20 '20 at 14:50
  • 1
    @WXJ96163 perhaps so/sth has made you doubt it. Or maybe someone has been explaining sth and only they know whether you have understood. – Weather Vane Mar 20 '20 at 14:54
  • @WeatherVane Thank you. Would you please make up a context? For instance, I am explaining COVID-19 to Alice. In what situation, Alice would say "I don't think I understand" instead of "I don't understand"? – WXJ96163 Mar 20 '20 at 14:59
  • One reason is given in the comment from FumbleFingers: politeness. – Weather Vane Mar 20 '20 at 15:02
  • @WXJ96163, here's a fairly common scenario: my boss explains the requirements of a new project; from what he's said, it appears as if he's asking me to do something illegal/unsafe/unwise; but that can't be right, why would my boss tell me to do something like that? I say, "I don't think I understand (because if I do understand, then what you're telling me to do is illegal/unsafe/unwise)" – Juhasz Mar 20 '20 at 15:18

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