Comparing "zai" and "you"

再 is for a repetition still to come; 又 for one that already happened.

再 = again, not yet · 又 = again, already

How it works

我不能再喝了，你走后我又喝了两瓶 — I can't drink any more; after you left I had another two bottles.

English uses "again" for both, which is why this split has to be learned deliberately.

又 sentences normally close with 了; 再 sentences don't.

wǒ bù néng zài hē le
我不能再喝了。
I can't drink any more.
nǐ zǒu hòu wǒ yòu hē le liǎng píng
你走后我又喝了两瓶。
After you left I drank another two bottles.
tā zuó tiān yòu chí dào le, míng tiān bié zài chí dào
他昨天又迟到了，明天别再迟到。
He was late again yesterday — don't be late again tomorrow.