What determines whether you use tidak or bukan?
Bukan negates nouns (Is this an apple / Is this your apple). Belum negates time events ... literally means 'not yet' as opposed to 'no' which would require 'tidak'. Other that nouns and 'not yet', tidak is used to negate everything else.
Do we "makan" sup or "minum" sup?
Might be a silly question, but I was always taught it was 'sop', i assume sup and sop are interchangeable for soup?
yes, both sup and sop means soup