The word 难忘 is formed by directly combining the concept of "difficult" (难) with the action of "forgetting" (忘). It creates a single, fused idea where the difficulty is intrinsically attached to the act of forgetting, resulting in the meaning of "unforgettable". It is not a temporary state but a permanent description, picturing a memory or experience that has etched itself so deeply that to remove it would be an inherent struggle, thus forming a compact unit for a lingering, indelible impression.