She criticized his carelessness mildly to avoid hurting his feelings.

mildly 是形容词 mild 的副词形式，mild 意思是"温和的、轻微的"，加上 -ly 变成副词后表示"温和地、轻微地"。 这里 mildly 修饰动词 criticized，表示"用温和的方式批评"。