Learn Hardcore Japanese: watasi ha piano wo hikuno ga zyouzu desu.

The の in 弾くのが上手です is a nominalizer. It turns the verb phrase ピアノを弾く (“to play the piano”) into a noun-like concept (“playing the piano”). Once nominalized, you can treat it like a subject and attach が before 上手です to comment on that activity.

Why does ピアノ still take the object marker を inside the nominalized clause?

Even inside the nominalized phrase 弾くの, you must mark the direct object of the verb. Here ピアノを弾く makes it clear that the piano is what’s being played. Nominalizing with の does not remove the need for proper case marking within the embedded clause.

Why is が used after の, not directly after ピアノ or 私?

The が marks the subject of the adjective 上手. After nominalization, 弾くの becomes the “thing” that’s good at something. So 弾くのが上手です means “(One is) good at playing,” with 弾くの as the subject marked by が.

What’s the difference between using の and こと to nominalize the verb?

Both の and こと can nominalize, but:

の feels more conversational and personal.
こと is slightly more formal or neutral and often appears in writing.
You could say ピアノを弾くことが上手です, but it sounds a bit stiff. Learners and native speakers generally use の in everyday speech when talking about their own skills.

Why do we put は on 私 instead of が?

は marks the topic (“as for me…”). By using 私は, the speaker sets themselves up as the frame for the comment that follows—namely, their skill at playing piano. If you used 私が, you’d be emphasizing “I” as the new subject, which can sound too abrupt in this polite, self-introducing context.

What part of speech is 上手, and how does it connect with です?

上手 is a な-adjective (also called an adjectival noun). To make it polite and complete the sentence, you attach です after it. So 上手です means “is skilled” or “is good at.”

Can I just say ピアノが上手です?

That phrase is technically understandable, but it’s not standard because you lose the verb “play.” It would literally mean “The piano is good/skillful.” Instead, to say “I’m good at piano” you either nominalize the verb (ピアノを弾くのが上手です) or use expressions like ピアノが得意です (“I’m proficient at piano”).

How would I make this sentence more casual?

In casual speech you can drop です and sometimes even 私 if context is clear:
• ピアノを弾くの、上手なんだ。
• ピアノ弾くのが得意。
Here なんだ or dropping です altogether makes it sound more informal.

Can I use this V-のが上手 pattern with other verbs?

Absolutely. This is a general pattern to talk about skills or talents. For example:
• 料理を作るのが上手です (“I’m good at cooking.”)
• 日本語を話すのが上手だね (“You’re good at speaking Japanese, huh?”)
Just replace ピアノを弾く with any verb you want to nominalize.