就 / 才【jiù / cái】

就 and 才 both go after a time, a number or a condition and before the verb. They carry the speaker's judgement: 就 says "already, sooner than expected", and 才 says "not until, later than expected". 他八点就来了 and 他八点才来 are both possible: the speaker's expectation decides.

了 goes with 就, not 才

就 marks something done early, and the sentence ends in 了. 才 marks something that was slow to happen, and takes no 了.

• × 他十点才来了 → 他十点才来
• × 我六点就起床 → 我六点就起床了 (for a past event)

Ages and amounts

The same contrast works with ages, durations and numbers of tries.

After a condition

After a condition, 就 says the result follows easily, and 才 says only that condition will do.

Each word has more uses of its own (就 "just", 才 "only" with small numbers); those are separate points.