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  1. I don't know what the cause is.
  2. I don't know what is the cause.

and do you have any difference between these sentences. if you have, could you explain it. e.g. 1) is for writing, 2) is for speaking

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I don't know what the cause is.

This is the correct one because you are not asking a question. You are just saying that you don't know what the cause is.

I don't know what is the cause.

This one is not correct. The inversion in the sentence is making it a question and in the meantime there is "I don't know" which doesn't have inversion and is not making it a question.

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    No, the inversion is not making it a question. Would it be a question to say "I don't know what is the cause of your acne?"
    – Robusto
    Feb 6, 2018 at 21:46
  • While I agree the first is more natural English, I agree with Robusto that the second is not a question, at least not as punctuated. "I don't know. What is the cause?" would be a question.
    – Andrew
    Feb 6, 2018 at 22:13
  • They mean the same thing. Neither strikes me an unnatural English. I have heard and used both.
    – JeremyC
    Feb 6, 2018 at 23:14

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