"Once a year" 这个短语通常不需要加 "for"。它本身就表示“一年一次”的意思。例如:
We celebrate the holiday once a year.
He visits his grandparents once a year.
如果你想要强调这是按照时间来安排的,也可以使用 "for":
We celebrate the holiday for one day every year.
He visits his grandparents for a week once a year.
但 "once a year" 本身已经足够传达这个意思,不需要 "for"。
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