When we ask an indirect, closed question we usually use if:
- Will he be home soon?
- Can you tell me if he'll be home soon?
- 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?
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?
We don't use if with "do you think" because think is an unstable information unlike know.