The old man furrowed his brow, his face marked by years of hardship.

furrowed 是动词 furrow 的过去式。furrow 本意是"犁沟、皱纹"，作动词时表示"使起皱纹"，常用来形容皱眉的动作。这里指老人因思考或情绪波动而皱起眉头。