"I am reading the book."
Translation:Saya membaca buku itu.
Aku is informal and should only be used between friends and family. If you use aku with strangers it implies superiority; not something you want to do ... especially if you are a foreigner. Stick with 'saya' whenever in doubt. The same applies to using -ku as a suffix. Again, avoid this with strangers and colleagues who are not close friends.
I'm confused because the sentence "Saya membaca buku" was not accepted though it's "the book", not "that book" in the english sentence...