He spent the whole day fine-tuning his guitar until the sound was perfect.

fine-tuning 是由形容词 fine（精细的）和动词 tune（调音、调整）组成的复合动词，加上 -ing 后缀构成现在分词形式。fine-tune 字面意思是"精细调整"，指对乐器、设备等进行细微调节以达到最佳状态。