What is なん in そうなんですか I understood roughly what sou desu, sou des ka mean. But I'm having difficulty in understanding the word nan in sou nan des ka. If Sou des ka can literally be translate as "correct/it seem so is it?" How does adding nan "what" fit in this sentence? If anyone could explain would be much appreciated.

This is not ; this is + +

is treated like (but it's really not) a na-adjective. So following the construction here, it becomes From the same website:

> (ndesu) has the same meaning as (desu), but is different in that it feels a bit more personal since it is used to explain something, give reasoning, or to emphasize something.

So corollarily, is like but it feels a bit more personal.

(Note: I believe this has already been answered somewhere in this site, but I'll answer it anyway for good measure.)