再 vs 又
Both mean "again". One looks ahead, the other looks back.
Both can be translated as "again", but the time direction differs.
再: it has not happened yet
Use 再 for a repeat you want, plan or request in the future.
Please say it once more.
。
Come again tomorrow.
又: it already happened
Use 又 for a repeat that has already taken place (or a habit that keeps recurring).
，又 。
He came yesterday and came again today.
You're late again!
Rule of thumb
Future or wish: 再. Past or already-true: 又.