Need help with a sentence

[Harv]

どうして日本語が上手なんですか

I know this is really basic but I'm not completely sure what it means.

I'm guessing it roughly means 'how come you are so good at Japanese?' right?

the nan at the end is confusing me a bit.

[Jason]

You're right. Though I wouldn't say "so good" but just "good."

The な here is just the normal na-adj な that you use when you put a na-adj before what it modifies. んです is short for のです. The のだ/のです construction is used to explain the reason for something. It translates literally into something like "it is that..." When used in a question like this, it's really doesn't change the meaning of the sentence. It just adds reinforcement that the statement you're asking about is true. This is very subtle, so let's look at a couple of sentences and a more literal translation of them:

どうして日本語が上手ですか？ - "Why are you good at Japanese?"
どうして日本語が上手なんですか？ - "Why is it that you're good at Japanese?"

These both mean the same thing, and the difference between then in both English and Japanese is very subtle. The first sentence is a simple question. The 2nd one takes "that you are good at Japanese" as an established fact.

That all being said, the real difference between the なん version and the other is very, very small. I wouldn't sweat it too much. Just think of it as an emphasizer.

[Harv]

Thanks for the help Jason-san .