Saya belajar gitar setiap sore dengan guru musik.

Saya is the formal or neutral first-person pronoun “I.”
Aku is more informal/intimate.
Which one to use depends on context and your relationship with the listener:

• Saya: polite, formal, everyday public speech
• Aku: casual, used with friends, family, peers

Indonesian does not use definite or indefinite articles. Nouns stand alone.
If you need to emphasize a or one, you can add:

• sebuah gitar (a guitar)
• satu gitar (one guitar)
  To emphasize the, context is enough or use a demonstrative:
• gitar itu (that guitar)

Indonesian generally follows Subject–Verb–Object, then time, then manner/companion:

1. Subject: Saya
2. Verb: belajar (to study/learn)
3. Object: gitar
4. Time adverbial: setiap sore (every afternoon)
5. Prepositional phrase: dengan guru musik (with a music teacher)

Yes. Time expressions are flexible. Both are correct:

• Setiap sore saya belajar gitar dengan guru musik.
• Saya setiap sore belajar gitar dengan guru musik.
  Moving setiap sore to the front can emphasize the routine.

guru by itself means “teacher” (of any subject). To specify the subject:

• guru musik = music teacher
• guru bahasa Inggris = English teacher
  You could drop musik if context makes it obvious, but adding it removes ambiguity.

Indonesian does not mark plural with word changes. You can clarify quantity by:

• Numerals: dua gitar, tiga guru
• Quantifiers: beberapa gitar (some guitars), banyak guru (many teachers)
• Reduplication: gitar-gitar, guru-guru (less common in everyday speech)

Absolutely. Just swap gitar and, if you like, specify the teacher’s specialty:

• Saya belajar piano setiap sore dengan guru musik.
• Or more precisely: Saya belajar piano setiap sore dengan guru piano.

Both refer to late afternoon/evening periods and overlap:

• sore typically around 3–6 pm
• petang leans more to 5–7 pm
  In everyday use, they’re often interchangeable, but regional preferences may vary.