才【cái】

才 in front of a number means "only": the amount, the time or the age is smaller than expected. The number can stand right after 才 with no verb.

With a verb

When there's a verb, 才 goes before it and the number follows the verb.

才八点 or 八点才

The position of 才 flips the meaning. 才 before the time says it's early; 才 after the time, before a verb, says something happened late.

No 了 at the end

才 + number says the amount is low. A final 了 says it has reached a point already, so the two clash.

• × 现在才八点了 → 现在才八点
• × 他才五岁了 → 他才五岁

Only a short time before something

才 + amount at the start, then 就, says something happened after very little.