就/才
jiù / cái
就 and 才 don't measure time. They measure your expectations
The picture A ruler of time with one tick on it: the point where you expected things to land. Finish before that tick and you get 就; land past it and you get 才. 已经 only reports that something has happened; 才 adds the sting of only now. 已经十点了 tells you the time, while 十点才到 tells someone off for being late.
Where the picture shows up in 就/才
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earlier than expected
他七点就到了。
tā qī diǎn jiù dào le
He was already there at seven.
Someone already in their seat at seven with the newspaper open, for an eight o'clock meeting
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later than expected
他十点才到。
tā shí diǎn cái dào
He didn't get in until ten.
Sitting in front of a dinner gone cold, and the door only opening at ten
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less effort than expected
我看一遍就记住了。
wǒ kàn yí biàn jiù jì zhù le
One read-through and I had it memorised.
Closing the book once, shutting your eyes, and reciting the page straight back
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more effort than expected
我看了三遍才记住。
wǒ kàn le sān biàn cái jì zhù
It took me three read-throughs to memorise it.
Turning back to the same page a third time before finally closing the book
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no gap between condition and result
你来了我就走。
nǐ lái le wǒ jiù zǒu
Once you're here, I'll be off.
Bag already in hand and half out of the chair the moment the door opens and they walk in
才 cannot stand here; 才 attaches only to what has already happened.
All of it in one picture
Being there at seven and getting it in one read-through both finish before the tick you had drawn — that is 就. Turning up at ten and needing three read-throughs both land past that tick — that is 才. In the last sentence the tick sits on a condition rather than a clock: between the moment you walk in and the moment I stand up there is no gap at all. Neither word is reading the clock; both are reading how far things fell from the tick your expectation put there.