"Saya membaca surat."

Translation: I read a letter.

[Johnathanv483403]

There are no definite/indefinite articles in Indonesia: "I read the letter" is just as valid as "I read a letter" and the translation will depend on context.

[Victoria534854]

Why not Aku. When to use say and when to use aku?

[DomErbacher]

Aku is the informal use. Use when around feiends or family.

Saya is the more formal term. Use this at school, work, etc.