When someone asks a question, is the reply, "That I don't know" correct? Or is the simpler "I don't know" a more correct or proper response?

  • Is "this I don't know" as idiomatic as "that I don't know"??
    – user10521
    Commented Sep 20, 2014 at 19:45
  • @nomadisland - Now that would make a good question. You should ask it as a new question. Feel free to refer to this question in a link, so that people know what inspired your question.
    – J.R.
    Commented Sep 20, 2014 at 21:40

2 Answers 2


Normally you just say:

I don’t know.

Sometimes when you want to draw attention to the thing unknown, you might venture to add an object:

I don’t know that.

But it would be rare indeed to apply inversion to put the now-important bit first:

That, I don’t know.

  • 12
    The emphasis on that in That, I don't know could make sense in a string of questions where someone was knowledgeable in answering the first few questions but not the last one. Usually then I would say, "Now that, I don't know," with spoken emphasis on that. Commented Feb 26, 2013 at 14:42
  • 6
    That I don't know is not unusual in BrEng. Commented Feb 26, 2013 at 15:36
  • 1
    If you are going to add "that", I would prefer "I don't know about that.".
    – user485
    Commented Feb 26, 2013 at 17:49
  • @BarrieEngland It isn’t unusual to me either. And for that matter, I only added the comma so people didn’t think it was the unstressed relative pronoun instead of the stressed demonstrative one. There is some sort of bias here, like recency bias but for locality, that I keep seeing and don’t have a word for. Actually, it isn’t even locality; it is merely experience — by which of course I mean inexperience.
    – tchrist
    Commented Feb 26, 2013 at 22:22

I disagree with tchrist on "That I don't know" being unusual. It is very common but has a different connotation. Saying "That I don't know" would be said when you had already answered a question or two, but did not know the answer to the current question. As in

What time are we leaving?

Five o'clock

Who's driving?

I am.

Is Barry going with us?

That I don't know.

  • when you had already answered a question or two... and knew the answers to the first few questions. The "that" emphasizes that the current question is unknown (when the others weren't). Commented Jan 27, 2014 at 10:37
  • This implies you can easily add 'thing' and make it "That thing I don't know", isn't it, guys?
    – Arman
    Commented Apr 24, 2015 at 13:30
  • 1
    That would be very awkward, at least in AE. You would be more likely to hear something like "That is something I don't know"
    – Kevin
    Commented Apr 25, 2015 at 15:01
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    @Arman No, it doesn't (at least not in AmE). "That thing I don't know" isn't idiomatic like "That I don't know" is.
    – cpast
    Commented Apr 27, 2015 at 23:52
  • 1
    @ArmanMcHitaryan "That one I don't know" is definitely less awkward than "that thing" but probably still not as common as just "That I don't know."
    – Kevin
    Commented Apr 28, 2015 at 13:16

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