才【cái】

才 goes before the verb and says the action came later than expected, or only after something else had happened. The sentence has no 了.

No 了 after 才

才 already says the action finally happened, so 了 is left out.

• × 他十点才起床了 → 他十点才起床

Only after something else

A first action or condition before 才 says the second happened only once the first did.

才 or 就

The same facts take 就 when they seem early and 才 when they seem late. 就 keeps 了; 才 drops it.

才 + number for "only"

Before a number, 才 says the amount is small – smaller than it should be, or than you'd think.