再 vs 又 (again)
副词 / adverbs (频率)
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
再 + V (future) / 又 + V + 了 (past)
Both mean "again", but:
再 = repetition still to come: 下次再来 (come again next time), 请再说一遍.
又 = repetition that already happened: 他昨天来了，今天又来了 (he came yesterday and came again today). Usually with 了.
Exception: before 是, 要, 想, 可以, 会, 又 is used even for the future: 明天又是星期一了.
Examples
欢迎你下次再来！
Huānyíng nǐ xià cì zài lái!Come again any time!
他今天又迟到了。
Tā jīntiān yòu chídào le.He was late again today.
我们明天再聊吧。
Wǒmen míngtiān zài liáo ba.Let's talk about it again tomorrow.
明天又是星期一了！
Míngtiān yòu shì xīngqīyī le!It's Monday again tomorrow!
Common mistakes
✗ 昨天我再去了 → ✓ 昨天我又去了.
✗ 明天我又来 → ✓ 明天我再来.