对考试的担忧一直折磨着他。

The worry about the exam was gnawing at him all day.

gnawing 是动词 gnaw 的现在分词形式。gnaw 本意是"啃咬、啮咬"，常用来比喻"持续不断地折磨或困扰"。在这里 gnawing at him 形象地表达了担忧像被啃咬一样持续折磨着他的状态。