Yes, telah is the same as sudah but used in more formal situations. I tend to overuse it, since it's the one I see more often at work.
Yes, if you wanted you could analyse di- and mem- as showing object and subject focus respectively rather than as active and passive verb prefixes; sometimes one analysis is more useful, sometimes the other.
In my experience object-focus sentences are certainly more common than passive verb sentences in English, but they're not as common as subject-focus ones. Of course sentences with di- are not exactly the same as an English passive verb sentence, but in taking isolated examples they work well enough as equivalents.
oleh is obligatory when the agent is separated from the verb, e.g., Candi dibelikan baju oleh kakaknya. (Candi was bought a shirt by his sister), but optional where the agent immediately follows the verb, e.g., Baju dibeli Candi (the shirt was bought by Candi).
Baju membeli saya (The shirt bought me???) I ran this by a native speaker of Indonesian (in fact, the Candi who is becoming so familiar to our readers),who rejected it as not being a sensible utterance.
The most usual way to say what I assume you mean would be: Baju yang saya beli (the shirt (which/that) I bought). First and second person pronouns and pronoun substitutes come immediately before a passive verb (with or without being attached to the verb) and the di- prefix is not used. Third person pronouns can come after the verb, in which case the di- prefix is retained. baju yang dibelinya. (the shirt (which/that)he/she bought).
Bingley
