又 / 再 / 还
又: again (already happened); 再: again (future); 还: still, also, more
HSK 3HSK 3.0 · 3
又 + V (+ 了) (repetition that has already happened) | 再 + V (repetition that will happen) | 还 + V (still; also; more)
Decide by time. 又 is for repetition that HAS happened: 他又迟到了. 再 is for repetition that WILL happen or is requested: 请再说一遍, 明天再来. 还 expresses continuation ('still': 十点了他还在工作) or addition, and in future wishes: 我还想再吃一碗 — 还 goes before the modal verb, 再 after it. Exception: 又 can refer to predictable future events (明天又是星期一了).
HSK 3–4 gap fills often offer all three as options.
Examples
他昨天迟到了,今天又迟到了。
He was late yesterday, and he's late again today.
我没听清楚,请你再说一遍。
I didn't catch that; please say it again.
这个电影太好看了,我还想再看一遍。
This movie is great; I want to watch it again.
十点了,他还在工作。
It's ten o'clock and he's still working.
明天又是周末了!
Tomorrow is the weekend again!
Exercises
1.他上星期来过,昨天 来了。
2.今天太晚了,我们明天 谈吧。
3.这个菜真好吃,我还想 吃一碗。
4.已经十二点了,他 在看书。