"inside" As Adverb And Question Tags

[Jesule]

Hi, how is it going? My first question is about "inside". I don´t know if it´s well-used in this sentence: "I want to see what is inside". For instance in a closed box. I know it´s right when I use the noun: "I want to see what is inside the closed box", but without the name, is it correct?. It´s supposed I used "closed box" in a sentence before. On the other hand, I always hesitate if i need a subject before the verb, "to be" in this case. "I want to see what "it" is inside". Is it right?. I am afraid I don´t need it. If i use "it", it would be incorrect, not?. I am thinking about a similar sentence now: "I don´t know what is going on". There´s no "it" before "is", and it is right, not?. I can´t imagine "I don´t know what "it" is going on". In this kind of sentences, don´t we need to put a subject in the subordinate clause? Why?
About question tags. Imagine this sentence, "You are telling lies". At first, I thought of "Isn´t it true" in order to say that someone is really telling lies, but then I realized I could use a question tag. "You are telling lies, are you not?". Can I use both? I mean "Isn´t it true" and "Are you not?". Does it mean the same thing?.

Thanks a million for replying me last time!
Learning English is really wonderful!

Jesús

[Casiopea]

[1] 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.

[2] 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.

[3] I don´t 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.

[4] I don´t know what "it" is going on".
=> "what" is the subject. Every verb should have one subject only, not two.

[5] You are telling lies, are you not?
OR
[5] You are telling lies, aren't you? <that one is a contraction>

[6] Isn't it true that you're telling lies?
[7] You are telling lies. Isn't that true?

[8] 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.