"I started my letter."
Translation:Saya memulai surat saya.
Ah - I actually found out why it's wrong - from another thread on another question. Basically - you shouldn't mix "Saya" and "Aku" in the same sentence. So if you start with saya you should finish with saya!
E.g. "saya memulai surat saya"
or "aku memulai suratku"
Also - from what my girlfriend tells me, no one really uses saya much in informal speech - so I've been trying to stick to aku where possible during my studies.
Thanks for your comment - I hadn't actually made the connection between Anda and Kamu being formal and informal versions of you. That's one of the things missing from Duo lingo - i.e. clarifications of some points like that. I still really love duo lingo though! I vaguely remember there were some of those points on the French course - perhaps it will come later to Indonesian (its only still in beta).