Let me help you with this question: “jiu” and “cai” are both used to indicate short time. but “cai” is used for past tense while "jiu"is used for future tense, for example:

nǐ shén me shí hòu lái? = when will you come?

wǒ jiù lái = I’m coming soon

nǐ shén me shí hòu dào de ? = when did you arrive?

wǒ cái lái/ dào = I just arrived

So here you can see, this cai means just, and “jiu” could be used as only for example:

who ate the bread? Is that you, Benny? (with angry tone)

bù, bú shì wǒ . wǒ jiù chī le 2 piàn miàn bāo = no, it wasn’t me, I only had 2 pieces of bread

Besides this, “jiu” can also be used to emphsize the object, for example:

nǐ hǎo, wǒ zhǎo Benny = hello, I’m looking for Benny

wǒ jiù shì = I am