才 + Number (only — a small amount)

Only ten years old

• A boy at the piano competition plays better than adults, and everyone is surprised by his age.
• he / him他tāonly才cáiten十shíyear岁suì，piano钢琴gāng qín就弹jiù dàn(degree marker)得deso much这么zhè megood好hǎo！ — "He's only ten, and he already plays the piano this well!"

only才cái + Number

• only才cái + number (+ measure word) = "only": the speaker finds the amount small. only才cáifive五wǔyuan块kuàimoney钱qián, only才cái两个liǎng gèperson人rén.
• Before a verb with a number, only才cái goes before the verb: I我wǒonly才cáito shout喝hē(completed / change)了le一杯yī bēi — "I've only had one glass."

this这zhè(measure word)件jiànclothes衣服yī fuonly才cáififty五十wǔ shíyuan块kuài。

Small amounts, early times

• I我wǒtalent and learning才学cái xué(completed / change)了le三个sān gèmoon月yuèChinese language中文zhōng wén。 — "I've only studied Chinese for three months."
• With a clock time it means "it's only …": now现在xiàn zàionly才cáiseven o'clock七点qī diǎn，a little more再zàisleep睡shuìa while一会儿yí huìr(suggestion)吧ba。 — "It's only seven; sleep a little longer."

now现在xiàn zàionly才cáiseven o'clock七点qī diǎn，a little more再zàisleep睡shuìa while一会儿yí huìr吧ba。

Which only才cái is it?

only才cái versus only只zhǐ

• only只zhǐ limits an amount neutrally: I我wǒonly have ...只有zhǐ yǒuten十shíyuan块kuàimoney钱qián.
• only才cái adds the speaker's surprise that the amount is small: he / him他tāonly才cáiten十shíyear岁suì — "He's only ten (so young!)."

I我wǒonly只zhǐhave有yǒuten十shíyuan块kuàimoney钱qián。

Recap

Read the full reference: HSK 3 grammar — Adverbs.