就 (ji`u/ then) and 才 (cái/ then) are both adverbs which are placed in front of verbs. The functional difference between ji`u (就) and cái (才) is that ji`u imparts a sense of earliness, scarcity, and easiness, while cái indicates a sense of lateness, ampleness, and difficulty in sentences. Below are a few examples illustrating the contrast between the two adverbs:
tā sān diǎn jiù dào le
He arrived at three o’clock, (which is earlier than expected.)
tā sān diǎn cái dào
He arrived at three o’clock, (which is later than expected.)
liǎng gè jiù gòu le
I need two of them. (The quantity is less than what the speaker or the listener expected.)
liǎng gè cái gòu
I need two of them. (The quantity is more than what the speaker or the listener expected.)
nǐ qù ，wǒ jiù qù
I will go there if you do. (It is easier to convince me than you expected.)
nǐ qù ，wǒ cái qù
I won’t go there if you don’t. (It is harder to convince me than you expected.)
Furthermore, ji`u (就) is commonly used in sequential narratives, such as the following sentence: 他來了以後，就先坐下，然後就說 (he came, then he sat down, then he said).
On the other hand, cái (才) normally signals a counter-expectation in sentences. For instance, in the sentence 才不是這樣 (That is not what happened.), cái (才) indicates that the speaker is about to raise a counter-argument.