The Chinese idiom 欲壑难填 is formed by combining four characters: 欲 (desire), 壑 (ravine), 难 (difficult), and 填 (to fill). Literally, it describes a ravine of desire that is hard to fill, creating a metaphor where insatiable human greed is likened to a deep valley that cannot be satisfied. The characters are arranged in a subject-predicate structure where 欲壑 (desire ravine) serves as the subject, and 难填 (hard to fill) acts as the predicate, directly conveying the concept through this figurative construction.