When we ask an indirect, closed question we usually use if:

  1. Will he be home soon?
  2. Can you tell me if he'll be home soon?
  3. Do you know if he'll be home soon?

Why do we not use if with do you think?

Do you think he'll be home soon?

  • I didn't add a question mark in the questions 2 and 3, because it's your question, I only corrected formatting. But I think there should be one. – Alenanno Sep 14 '11 at 13:40

Because formally the question is not about whether he is coming, but about what you think.

Pragmatically, it usually does have the same meaning as the other examples, but with a different embedded question that may not be so:

Do you think I should go?

  • I'm not convinced that "formally", "do you know if" is any more or less about what you think than "do you think". Note that you can reasonably say, for example, "Do you know he'll be home soon?" (with emphasis on "know"). That question explicitly asks whether you're certain of what you think (but only assuming you do in fact at least think he will). I'm not sure exactly how to formally distinguish that question from OP's example 3 (with no emphasis, OP's capitalisation notwithstanding). – FumbleFingers Sep 14 '11 at 15:17
  • 5
    I agree that a "do you know" question is formally a question about the mental state of the interlocutor, just as a "do you think" question. But in the case of "know", the expressions "know that" and "know if/whether" both exist, and have different meanings, so "do you know that" and "do you know whether" questions have different meanings. In the case of "think", we do not have the expression "think if" or "think whether" but only "think that". Perhaps that difference is what the question is really about. – Colin Fine Sep 14 '11 at 16:48

We don't use if with "do you think" because think is an unstable information unlike know.

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