phrase
A2
used to talk about something that happened in the past
One day, I came home to find my windows smashed.
有一天，我回到家发现我的窗户被砸碎了。
B1
at some time in the future
I'd like to go to Berlin again one day.
我希望有一天能再去柏林。
translation
有一天, 将来有一天