"tau" is not really an indonesian word, it's only used in informal conversation
terima kasih, 1CN5ijC. but my Indonesian fiancé taught me Tau and not Tahu. i will ask her what the difference is here as she also tells me Saya is used more in Melayu than Indo. hmmm
You shouldnt worry about it. You can just say and write tau as many other people do. A general rule of linguistics is - today's most frequent mistakes are tomorrow's rules. There's lots of variety and many dialects in the Indonesian language.
"Tahu" is the formal form, as people usually say it "tau" without the "H" (This is a wrong habit).
Saya is often to use in formal conversation or when you're speaking to people that older and you don't know (Even in informal conversation)