She set it beside her husband's picture and looked at it thoughtfully.

thoughtfully 是形容词 thoughtful（沉思的、体贴的）加上副词后缀 -ly 构成的副词形式。thoughtful 由名词 thought（思考）加上形容词后缀 -ful（充满...的）组成，字面意思是"充满思考的"。thoughtfully 在这里表示"若有所思地、沉思地"，描述一种带着思考或回忆的状态。