Question

He often washes his face once a day.中的wash到底用不用s形式

Answer 1

often、always、usually为频率副词，通常用在一般现在时中。一般现在时中的动词要注意第三人称单数，通常情况下为动词原形+S，特殊的(wash,
watch)+ES。所以答案是washes

Answer 2

用。你的老师以前教的可能是过去常常发生的事情，那样应该用过去式。如;

He
often
washed
his
clothes
last
year.