Both 才 and 就 can be used as an adverbial modifier before a verb
就 indicates that in the speaker’s opinion, the action happened early, or went on fast or smoothly. For example:
Why are you going to bed so early?
你怎么这么早就 要睡觉了？ （earlier than expected time）
Nǐ zěnme zhème zǎo jiù yào shuìjiàole?
才 indicates that in the speaker’s opinion, the action happened late, or went on slowly or unsatisfactorily. For example:
Class begins at seven o’clock, but he doesn’t get up until eight o’clock.
七点上课，他八点才 起床。（later than expected time）
Qī diǎn shàngkè, tā bā diǎn cái qǐchuáng.
Welcome to have my 1-ON-1 lessons on http://www.verbling.com/teachers/dawei 🙂