Learn Portuguese (Portugal): O piano é um instrumento muito bonito. - The piano is a very beautiful instrument.

We use ser for permanent qualities, definitions, or inherent facts. Since being a beautiful instrument is an inherent characteristic of the piano rather than a temporary state, é is the correct choice.

Why are the words in the order instrumento muito bonito instead of "muito bonito instrumento"?

In Portuguese, adjectives generally go after the noun they describe. So, "beautiful instrument" translates to instrumento bonito. The adverb muito (very) goes right before the adjective it intensifies, resulting in instrumento muito bonito.

If I were talking about a feminine instrument, like a harpa (the harp), would bonito change to bonita?

No, it would stay exactly the same: A harpa é um instrumento muito bonito. Adjectives always agree in gender and number with the noun they directly modify. Here, bonito modifies the masculine noun instrumento, not the subject of the sentence.

Does the word muito ever change its ending to agree with the noun?

Not in this sentence! When muito means "very" and modifies an adjective (like bonito), it acts as an adverb and is completely invariable (it never changes). It only changes to muita, muitos, or muitas when it means "a lot of" or "many" and modifies a noun directly.