both aku and ku-/ku- are pronouns. "aku" is written separately with the word that follows it. for example: aku mengerti (I understand), ibu aku (my mom)."ku-" or "-ku" is written as a series of the word that follows it or as a series of the word followed by it, in other words u have to connect it with other word. for example: kumengerti bahwa.. (I understand that..), setahuku (as I know), ibuku (my mom). I hope u can understand that :)