‘cái’ signals necessary condition, while ‘jiù’ signals sufficient condition, for instance:
Tiānqì zhuǎnqíngwǒmen jiù dào hǎibiān qù kǎoròu.
(As soon as the weather turns clear, we’ll go to the seashore to have a barbecue.
Duō shuō, duō liàncáinéng shuōde hǎo Zhōngwén.
(You can learn to speak Chinese well only by speaking more and practicing more.)
In (1a) the event of ‘going to the seaside for a barbecue’ is contingent on the weather turning clear’. In (1b), achieving the result of speaking Chinese well is conditioned on ‘speaking more and practicing more’. The difference between (1a) and (1b) is that ‘cái’ indicates necessary condition while ‘jiù’ indicates sufficient condition.
 Loar, J. K. (2011). Chinese syntactic grammar: functional and conceptual principles. New York: Peter Lang.