some day原意“若干天”,引申义"来日"(将来的某一天),或"总有一天".常用于将来时,如:
Some day my prince will come.总有一天我的王子会出现.
someday 意为"有一天",多指将来的一天.在这一点上,someday=some day.就是说上面的例子完全可以换成someday.
其实意思都差不多,只是someday 多放于句首,
eg.
Someday,if he gets many more customers,Billy might win a prize for being an outstanding newspaper boy.
也许有一天,如果他能争取到更多些顾客,他会获得优秀报童奖.
来自于番茄小精灵的答案