才 vs 就
cái vs jiù
Comparing "cai" and "jiu"
才 says it took longer or more effort than expected; 就 says it took less.
才 = later / harder than expected · 就 = sooner / easier
How it works
我花了两天才修好，你一下子就弄坏了 — it took me two days to fix, and you broke it in an instant.
The pair encodes the speaker's expectation, not objective time. The same clock time can take 才 or 就 depending on what was anticipated.
A practical tell: 就 sentences usually end in 了, and 才 sentences don't.
Examples
wǒ huā le liǎng tiān cái xiū hǎo
我花了两天才修好。
It took me two whole days to fix it.
tā yí xià zǐ jiù nòng huài le
他一下子就弄坏了。
He broke it in an instant.
wǒ bā diǎn jiù dào le, tā jiǔ diǎn cái lái
我八点就到了，他九点才来。
I arrived at eight; he didn't turn up until nine.