The blacksmith nailed new horseshoes onto the horse.
Tips
memory
马 (horse) + 蹄 (hoof) + 铁 (iron) - literally 'horse-hoof iron.' The shape of a horseshoe is also called U-shaped (U形) in Chinese.
culture
While Western traditions hang horseshoes above doors for luck, this is not a traditional Chinese custom. The superstition is mentioned mainly in the context of Western culture.