Using verb + "n desu", "n da" in questions and in statements and with no confirmation

Question

Are the following phrases exactly the same in terms of meaning? And are they all correct in terms of grammar? Can I use them interchangably?

• tokyo e iku
• tokyo e iku n
• tokyo e iku no da
• tokyo e iku no desu
• tokyo e iku nda
• tokyo e iku ndesu

I know that, for example,

tokyo e iku ndesu ne?

can be used to mean "you're going to Tokyo, right?" -- just to confirm what you already know.

But I've heard "n desu" used in normal questions, where no confirmation is seeking by one asking a question.

nani o suru n desu ka?

Why is that?

Answer 1

東京へ行く and 東京へ行くんだ (which is essentially equivalent to 東京へ行くのだ) are quite different in nuance. The first is merely expressing the proposition of going to Tokyo. The second asserts that it is the case of a situation of going to Tokyo, in a way similar to 本だ asserting that it is the case that something is a book. Literally, it could be glossed as "It is that I/you/he are going to Tokyo". Colloquially, assuming a bit of context, it could conceivable be translated as "So you're going to Tokyo, huh."

With regard to なにをするんですか, the same applies. Whereas なにをしますか merely asks what you are doing or will do, なにをするんですか again literally could be rendered as "What is it that you are doing", which has the possible nuances, just like in English, of "What on earth on you doing", or "Why are you doing that".

Answer 2

First of all, "no" and "n" are same. "no" can become n if there is no stress.

"no" is called 準体助詞, which makes a noun phrase from a sentence / an adjective. It is just like "that" in English. "no" itself does not mean confirmation or question. And, rather what comes after "no" adds these meanings.

~ndesu. It is that blah blah.

~ndesuka? Is it that blah blah?

~ndesune. It is that blah blah, isn't it?

Also, I want to note that ka at the last is sometimes omitted.

Answer 3

Basically, when you have an assumption about the response and are interrogating the other person, you append -n to the verb.

See here for more information.