Un evento musicale in giardino è interessante.

In Italian, un is the standard masculine indefinite article used before:

• All masculine nouns beginning with a vowel (like evento).
• Masculine nouns beginning with most consonants (except special cases).

uno is only used before masculine nouns that start with:

• s + consonant (e.g. uno studente)
• z, ps, pn, gn, x, y (e.g. uno zio, uno psicologo)

Since evento begins with a vowel e, you use un evento.

In Italian, descriptive adjectives normally follow the noun they modify. So you say:

• evento musicale (noun + adjective)
  Putting musicale after evento follows this default order.
  Some adjectives can shift before the noun for emphasis or style, but musicale is almost always placed after.

Italian often drops the article when talking about being in certain common places or rooms:

• a casa (at home)
• in chiesa (in church)
• in giardino (in the garden)

Using nel giardino (in + il + giardino) would refer to a specific garden already mentioned, rather than speaking of the action "in the garden" in general.

Use nel giardino when you mean a specific garden:

• “I bambini giocano nel giardino della scuola.”
  Here you’re specifying which garden.
  For a generic location or common expression, stick with in giardino.

When linking a noun to an adjective with essere (“to be”), you do not use an article before the adjective.

• Correct: L’evento è interessante.
• Incorrect: L’evento è un interessante.

If you add an article, it implies you’re turning the adjective into a noun (e.g. un interessato = “an interested person”), which changes the meaning.

Yes. Interesse (noun) ↔ interessare (verb) works differently:

• Essere interessante: describes something’s quality.
  • Un evento musicale in giardino è interessante.
• Interessare: requires an indirect object.
  • Un evento musicale in giardino mi interessa.
  (“A musical event in the garden interests me.” / “I find it interesting.”)

Change both the noun and adjectives to plural:

• Eventi musicali in giardino sono interessanti.
  If you include the article:
• Gli eventi musicali in giardino sono interessanti.

Both are correct but slightly different in style:

• Un evento musicale uses the adjective musicale (“musical event”).
• Un evento di musica uses a noun phrase (“an event of music”).
  The first is more compact and common; the second emphasizes the idea of music itself.

Yes, a common alternative is all’aperto:

• Un evento musicale all’aperto è interessante.
  You could also specify with an article:
• Un evento musicale in un giardino (in a garden)
• Un evento musicale nel giardino comunale (in the municipal garden)