what express can I use to talk about the past?
I understand that "one day" can be used to say "a day in the future" . For example,
I would like to travel to the moon one day.
but what expression I can use to say something in the past, not a specific time. For example,
One day, when I was walking along the street, I came across a friend of mine.
can I use "once" ? but it seems that my dictionary does not say that's acceptable!