Difference between kara and n desu?


Is there a difference (or nuance) or is it just two ways of saying the same
thing?

For example, どうして、とぶか？ （１） こわいんです。 （２） こわいですから。

Obviously, those are very simple examples. But are there times where you
couldn't use one in place of the other to explain "why" you did something (or
you did something "because")?




「んです」,「んだ」and friends give a nuance of conveying new information or explaining
something to someone for the first time.

「からです」etc. is just a reply to some explicit question of why.

For example, something like this conversation might have both with very
different meanings/nuances:

> さち：dotnetN00b、もう退社してる？[dotnetN00b, you're already leaving work?]
>
> dotnetN00b: ううん。サーティワン行くんだよ。ついてくる？ [No, I'm going to Baskin Robin's! Wanna
> come?]
>
> さち：いきなりどうして？ [Whoah, that's out of the blue. Why you going?]
>
> dotnetN00b: サーティワン、マージうまいからだよ！毎日食っちゃってる。ふふん。 ['cause Baskin Robin's is
> ridiculously good! I go there every day. :P]

Clear as mud?

