a blue moon的用法和樣例：
例句
- Barry only visits his parents once in a blue moon.
巴里很難得去看望他父母一次。 - If the river overflows its banks, the garden gets flooded; but that only happens once in a blue moon.
河水一旦泛濫，這片園子就會淹沒，但這種情況極為罕見。 - I see her once in a blue moon.
我難得看到她。
詞匯搭配
- once in a blue moon 罕見的，極少的，難得...
