hisraemさん、エリックさん、マイケルさん、
こんにちは!
Thank you very much for brilliant helps, エリックsan and マイケルsan!!
Like マイケルsan wrote, the question regarding ~んです is difficult and it's quite a common question, so
hisraem-san, it's not a silly question at all!
It's actually difficult to explain too, so I also found エリックsan't explanation very helpful.
As エリックsan wrote with an example conversation, "obviously they're already here" is an important point.
When we talk something obviously known, we add ん (or の in more formal expression) and express "we know
this is a obvious information, but..." kind of feelings or certain surprise.
In 「いつからここにいるんですか」, the sense/feelings behind the scean would be "I know you obviously know that
I'm here now, but since when have I been here?" and/or "I'm here for some time? Why am I even here? Since when?"
In this case of いつからここにいるんですか, it could be both asking for explanation AND certain surprise.
When we ask questions which contains some information that the hearer obviously knows,
this ん adds "courtesy" and "respeact" of "I know you obviously know that ...., but" so that the hearer
won't feel "offended" or say "I know that".
Hope this helps too
Natsuko(奈津子),
Team JapanesePod101.com