One day既可指过去的某一日,也能指将来的某一天.只不过用以指过去的场合比用以指将来的场合要多得多.
Some day只能用以指将来,不可以指过去.例如,我们可以说I hope to see him again some day 但不能说I saw him some day.例：
If you don't change,you will be sorry for it some day．
如果你不改变,将来有一天你会为此而感到遗憾的.