2. I don't know what is true and what is not.
Can I also say: I don't know what is the truth and what is not. <<
This is almost correct, but not quite. It's that noun clause thing again. Remember that?
You've written what should be a noun clause as an interrogative, which means you've inverted the subject and the auxiliarly verb "be". A noun clause does not invert the subject and an auxiliary verb. The order remains in statement form: what the truth is - not "
what is the truth".
This is how the sentence is correct: I don't know what the truth is and what is not.
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