再 / 又 / 还【zài / yòu / hái】

再, 又 and 还 can all mean "again" and all go before the verb. The split is whether the repeat has happened: 又 for a repeat that is real, 再 for one that isn't yet, and 还 for one asked about or intended.

又 – it has happened

又 usually takes 了 and often a note of annoyance. It also covers future events that are certain to come round – a weekday, a holiday, an exam – with 是, 要 or 该 and a final 了.

• × 他昨天再来了 → 他昨天又来了

再 – it hasn't happened

再 is for requests, plans and conditions. It follows a modal verb: 想再, 要再, 能再.

• × 我再想去 → 我想再去

Under 没, 再 can describe the past: the repeat didn't happen.

还 – asked about, or wanted

还 does the job of "again" in questions about the future and before a modal verb – the opposite position to 再.

我想再去一次 and 我还想去一次 mean nearly the same; 还 adds that you've been before and want more. 还 can't take 了 with this meaning.

再 or 又 in the same sentence

Asking for a repeat uses 再; complaining that it happened uses 又.