one day 既可以用于过去时,也可以用于将来时.例如：
I will take you there one day.总有一天我会带你到那去的.
One day I met him in the street.有一天我在街上遇见了他.
而some day则只能用于将来时.例如：
Some day you will be sorry about it.总有一天你会对这件事感到后悔的.
He put away those iron scraps,hoping that they may come in useful some day.他把那些碎铁皮收藏起来,希望这些东西也许有一天会有用处.