- I don't know what the cause is.
- 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.