When her children refused to do their homework, she had to put her foot down.

put 是动词，意思是"放置"，在这里是短语动词的一部分。 her 是代词 she 的所有格形式，意思是"她的"。 foot 是名词，意思是"脚"。 put one's foot down 是一个英语习语，字面意思是"把脚放下去"，实际含义是"坚决反对"或"采取强硬立场"，表示当某人决定不再容忍某种情况时采取的行动。