2. In a broad sense, "n desu" is used when making in enquiries:
ashita doko he iku n desu ka.
Where are you going tomorrow?
You can also easily say "Ashita doko he ikimasu ka" but its more neutral and so the "n desu" sentence has a certain amount of concern/emotion attached to it.
The "n desu" construct can also be used to give reason:
Boy: Kanojo wa kinou paatii ni kita no? (kita n desu ka)
Did your girlfriend come to the party yesterday?
Friend: iie, kaze wo hiichatte konakatta n da.
No she didn't come coz she caught a cold.
And it can be used perhaps emphatically too:
Boy (surprised): Sou nan da...konakatta n da
Oh wow I see... so she didn't come.
Of course its use is probably more complicated than this though. Textbook might be able to explain it better than I did
2. I have never heard of furuina tokei before heh. In any case the -na and -i adjectives for the most part remain mutually exclusive. As Glenn said before ookii and Chisai are the exceptions as they can take on -na adjective forms ookina and chisana but they are used in only very specific circumstances.
4. "de" can also be used to denote passages of time eg.
San jikan de nihongo wo benkyou shimashita
I studied Japanese for three hours.