Learn Hardcore Japanese: watasi ha piano ga suki desu.

Although it’s written with the hiragana は, when used as a particle it’s always pronounced wa. This is a historical reading preserved in modern Japanese particles.

What role does は play in this sentence?

は is the topic marker. It sets 私 (“I”) as the topic: “As for me, ….” This is different from marking the grammatical subject.

Why is ピアノ followed by が instead of を?

Because 好き isn’t a transitive verb but a na-adjective meaning “likable” or “be fond of.” The thing you like is marked with が, not the object marker を.

What part of speech is 好き, and how does it work grammatically?

好き is a na-adjective (形容動詞). Grammatically it behaves like a noun/adjective that needs a copula (です, だ) to form a sentence.

Why don’t we say 好きます if we want to express “like”?

Because 好き is not a verb, it cannot take the verb ending -ます. Instead, you attach the copula (です in polite speech, だ in plain speech) to complete the idea.

What is the function of です, and how would the sentence look in plain form?

です is the polite copula, giving a courteous tone. In plain (casual) speech you could say:

私はピアノが好きだ。
Or even just ピアノが好きだ。
You can also drop だ in very casual contexts: ピアノ好き.

How do you say “I like playing the piano” instead of just “I like piano”?

You need the verb 弾く (“to play”). Nominalize it with の and mark it with が: 私はピアノを弾くのが好きです。
(“As for me, playing the piano is something I like.”)

Can you omit 私は and simply say ピアノが好きです?

Yes. Japanese often drops the topic when it’s clear from context. ピアノが好きです naturally implies “I like the piano.”

How do you express “I don’t like the piano”?

You negate the copula or na-adjective:

Colloquial: 私はピアノが好きじゃないです。
More formal: 私はピアノが好きではありません。

What’s the difference between 好き and 大好き?

大好き (だいすき) adds the prefix 大 (“big”), so it means “to love” or “to really like” something much more strongly than plain 好き.