The word 不久 is formed through the direct and logical combination of its two constituent characters: 不 (bù), meaning "not", and 久 (jiǔ), meaning "a long time" or "duration". By placing the negative 不 before the concept of a long time 久, the compound naturally and conceptually creates the meaning of "not a long time", which inherently suggests a short or brief period, hence "soon" or "before long". Its formation is a straightforward semantic opposition, where the meaning of the whole is immediately derived from the clear relationship between its parts.