some day可以当成词组使用，意思有“总有一天”、“有朝一日”、“将来”，常用于将来时；而someday只能当单词使用，意思有“某一天”、“将来有一天”，常放于句首。
some day意为总有一天、有朝一日。常用于将来时。例句：Some day I'll pay you back for this!总有一天我会为此报复你。Some day someone will write the full epic of this exciting expedition.总有一天有人会把这部激动人心的远征史诗全部写下来。someday意为某一天、有朝一日，是一个副词，常放于句首。例句：We may find it useful in future.; It may come in handy someday.这东西日后可能用得着。I would like to go to college someday.我想有一天能上大学。Someday soon, however, this scenario could be real.有一天很快，但是这一个情节可能是真正的。
