Millie often (spends ) an hour a day (doing ) her homework.

spend ... doing sth. 花... 做某事
主语是 Millie 第三人称单数,动词也用三单,所以用spends


祝你学习进步,更上一层楼! (*^__^*)
不明白的再问哟,请及时采纳,多谢!