one day 和 someday 有什麼不同？

one day是過去或未來的某一天，要寫成兩個字

放在句首時後面要逗點

而someday則只能指未來的某一天，要寫成一個字

放在句首時後面不要逗點

比如：

She believes she will be famous one day.
= She believes she will be famous someday.
(她相信她有一天會很有名。)
(此時one day = someday，因為是未來的事)

Someday you’ll know what it’s like to be a parent.
= One day, you’ll know what it’s like to be a parent.
(有一天你會知道為人父母的感受。)
(此時one day也等於someday，因為仍是未來的事)

I ran into a high school classmate one day.
= One day, I ran into a high school classmate.
(有一天我巧遇高中同學。)
(此時就只能用one day，因為是過去的事)