Why it exists. Two small words that show how you feel about the time. 就jiù = earlier, faster or easier than expected. 才cái = later, slower or harder than expected. Mark uses both in Text 3: 银行九点才开门，我们八点半就到了yínháng jiǔ diǎn cái kāimén, wǒmen bā diǎn bàn jiù dào le.

Word	Means	Example
就	early	她七点就到公司了。 Tā qī diǎn jiù dào gōngsī le. · She was at the office by seven (that's early).
才	late	他十点才起床。 Tā shí diǎn cái qǐchuáng. · He didn't get up until ten.
就	fast, easy	这个菜很简单，十分钟就做好了。 Zhège cài hěn jiǎndān, shí fēnzhōng jiù zuòhǎo le. · It's easy: ready in just ten minutes.
才	slow, hard	那个菜不好做，两个小时才做好。 Nàge cài bù hǎo zuò, liǎng ge xiǎoshí cái zuòhǎo. · That one's tricky: it took two whole hours.

Key points

Both go before the verb (or before 能néng / 可以kěyǐ), after the time: 我们八点半就到了wǒmen bā diǎn bàn jiù dào le, 银行九点才开门yínháng jiǔ diǎn cái kāimén.
For the past, 就jiù usually has 了le at the end; 才cái doesn't: 他十点才来tā shí diǎn cái lái, not 他十点才来了tā shí diǎn cái lái le.
Same time, different feeling: class starts at nine and he arrives at eight: 他八点就来了tā bā diǎn jiù lái le. Class starts at seven: 他八点才来tā bā diǎn cái lái.
Not in the book · 才cái + a number can also mean "only": 他才十四岁tā cái shísì suì "he's only fourteen".

Common mistakes我昨天十二点才睡觉了。 → 我昨天十二点才睡觉。Wǒ zuótiān shí'èr diǎn cái shuìjiào. When 才cái means "late", no 了le at the end.
他就八点来了。 → 他八点就来了。Tā bā diǎn jiù lái le. 就jiù and 才cái come after the time, right before the verb.
坐地铁二十分钟才到了，真快！ → 坐地铁二十分钟就到了，真快！Zuò dìtiě èrshí fēnzhōng jiù dào le, zhēn kuài! Fast or early = 就jiù. 才cái means slow or late.