I want to see what is inside. <correct>
=> "inside" is a preposition, and its object "in the box" has been omitted for the sake of efficiency.
 I want to see what it is inside. <incorrect>
=> "what" is the subject of the verb "is". Adding another subject ("it") makes the sentence ungrammatical.
 I donīt know what is going on.
=> "what" is the subject. "what" replaces a noun, like this,
An apple was in the box.
I didn't know an apple was in the box.
I didn't know what was in the box.
 I donīt know what "it" is going on".
=> "what" is the subject. Every verb should have one subject only, not two.
 You are telling lies, are you not?
 You are telling lies, aren't you? <that one is a contraction>
 Isn't it true that you're telling lies?
 You are telling lies. Isn't that true?
 You are telling lies, isn't that true?
=> "isn't is true" is a sentence, not a phrase, so it can't function as a tag.
All the best.