Any help with this phrase?


> ジョンさんは日本語を **上手に** 話します。

The thing is that I don't understand why 上手 has the particle に. I'm reviewing
an examination sheet, and this is the answer to one of the questions. The only
possible thing that I can come off with is that the particle NI is there to
simply emphasize that John is good at speaking IN Japanese, although I don't
usually see 上手 paired with に. I usually see it with が.

Thanks beforehand :D




上手 is ordinarily a [形容動詞]{けいようどうし} or "na-adjective". It could be used as a
noun on its own, but more commonly it's describing something else.

Like with other 形容動詞, it takes な when describing another noun attributively -
just as it would be followed by だ・です to describe something predicatively (like
「彼は日本語（が）上手です」). But here, it is describing a _verb_ (話します, i.e. the formal
form of 話す) attributively, so it takes に.

This is common grammar; you may also be familiar with [静]{しず}かに "quietly", or
at least [本当]{ほんとう}に "truly", which is practically a set expression by itself.
This に is essentially the "-ly" part (disregarding of course that in English
we say "well" rather than "goodly").

Using が here would make no sense; it would have to mean something like "As for
John, his skill speaks Japanese". (Or his
[flattery](https://jisho.org/search/%E4%B8%8A%E6%89%8B), using an alternate
definition.)

