Restriction of scope vs emphasis on lateness
只 and 才 can both translate as English "only", but they do very different work. 只 limits the amount or type: 只有一个 (there is only one), 只买了水 (only bought water). 才 emphasizes that something required a long wait, much effort, or was less than expected: 八点才到 (did not arrive until eight, late), 三次才成功 (only succeeded on the third try). 只 is about scope; 才 is about timing or effort.
他学了五年才说得好,我只学了一年。
Tā xué le wǔ nián cái shuō de hǎo, wǒ zhǐ xué le yī nián.
He only spoke well after five years of study; I have only studied for one year.
只 = restricts quantity or type
只 precedes a verb or quantifier and narrows the range: 我只喝水 (I only drink water), 店里只有三个人 (there are only three people in the store), 只要十分钟 (only takes ten minutes). 只 is often paired with 就: 只要一小时就够了 (just one hour is enough). The "only" here is a scope restrictor; 才 cannot substitute.
才 = emphasizes lateness
才 highlights that an event happened later than expected or took more effort than anticipated: 他早上九点才起床 (he did not get up until nine, late), 学了三年才会 (took three years to learn). The speaker is either annoyed, impressed, or noting the difficulty. 才 is often paired with 就 in contrast: 八点就到 (as early as eight) vs 八点才到 (not until eight).
等了半个小时,他才回来。
Děng le bàn ge xiǎoshí, tā cái huílái.
I waited for half an hour, and only then did he come back.
才 = only with effort or condition
才 after a conditional clause emphasizes that the result requires the condition: 只有努力,才会成功 (only through effort can one succeed), 必须通过考试才能毕业 (you can graduate only after passing the exam). This "only if" construction pairs 只有 / 必须 / 要 with 才 to stress that the condition is required for the outcome.
只有付钱才能进去。
Zhǐyǒu fù qián cái néng jìnqù.
Only by paying can you go in.
The 只有...才... structure
只有A才B is "only A, then B" for essential conditions: 只有多练习才会进步 (only by practicing more can you improve). This is a signature Chinese rhetorical structure for cause-effect. The 只有 restricts the condition; the 才 emphasizes that the result comes only after the condition is met. Both words work together; dropping either weakens the construction.