Future repetition vs immediate sequence
再 points to a future repeated action: 再吃一口 (one more bite), 再说一遍 (say it once more). The action recurs or adds. 就 does not indicate repetition; it marks tight sequence or emphasizes immediacy. 吃完就走 (finish eating, then go right away), 回到家就睡 (get home, then sleep immediately). The 就 here is the "immediately after" connector.
吃完饭就出发,以后再讨论别的。
Chī wán fàn jiù chūfā, yǐhòu zài tǎolùn bié de.
Eat, then leave immediately; we will discuss other things later.
再 = future "again"
再 with a verb points to a future recurrence: 明天再去 (go again tomorrow), 有问题再问 (ask again if you have questions). This is the standard way to express "X again in the future". Unlike 又 (past repetition) or 还 (continuation), 再 is reserved for future actions that recur or add to something. Common in suggestions, instructions, and commitments.
如果你饿了,再吃一点。
Rúguǒ nǐ è le, zài chī yīdiǎn.
If you are hungry, eat a little more.
就 = immediate or tight connector
就 connects two clauses with "then, immediately, right away": 他一说完就走了 (as soon as he finished speaking he left), 下雨我就回家 (if it rains I will go home immediately). The 就 emphasizes that the second event follows the first without delay. Often paired with 一...就... (as soon as): 一下课就回家 (as soon as class ends, go home).
就 = "only / just" for quantity
就 before a number or quantity means "only, just": 就一个 (only one), 就两分钟 (just two minutes), 就这样 (just this way). This usage overlaps with 只 (zhǐ, only) but 就 has a slightly emphatic "exactly this and no more" feel. Both work in many cases; 就 sounds a bit more colloquial.
我就想喝一杯咖啡,不要别的。
Wǒ jiù xiǎng hē yī bēi kāfēi, bú yào bié de.
I just want to drink a cup of coffee, nothing else.
就 = "as early as / since"
就 before a time expression emphasizes that the time was early or that the event happened sooner than expected: 他五点就起床了 (he got up as early as five), 从小就喜欢 (has liked it since childhood). The 就 contrasts with 才 (cái), which emphasizes "not until / late": 他五点才起床了 (he did not get up until five, late). 就 early; 才 late.