就 (jiù) and 才 (cái) — speaker's stance on timing and difficulty
就 jiù and 才 cái are tiny adverbs that carry an enormous amount of speaker attitude. Both sit right before the verb. Both can be glossed loosely as 'then' or 'only', but they pull in opposite directions. 就 says 'sooner / easier / more readily than expected' — fast, smooth, on the early side. 才 says 'later / harder / only after some difficulty' — slow, with effort, on the late side. 他七点就来了。 = He came as early as 7 (sooner than expected). 他十点才来。 = He didn't come until 10 (later than expected). The verbs are identical; the speaker's view of whether it was early or late is what 就/才 carry. They also do other jobs (就 = 'right away / precisely', 才 = 'only / merely'), but the temporal-stance use is the heart of HSK 4.
Position — both sit right before the verb
Subject + (time phrase) + 就 / 才 + verb (+ rest). Notice the time phrase comes BEFORE 就/才, not after. 他七点就来了。 = He came as early as 7. 他十点才来。 = He didn't come until 10. 三岁就会说三种语言。 = (He) could speak three languages by age 3 (impressively early). 三十岁才结婚。 = (He) didn't get married until 30 (late by the speaker's lights). The position rule is iron-clad: time/condition first, then 就 or 才, then the verb. Native speakers parse the contrast almost entirely through this one-character switch.
就 — sooner / easier / more than expected
就 signals that the action came faster, more readily, or more straightforwardly than the listener might expect. (1) Earlier than expected: 我五点就起床了。 = I got up as early as 5. 她还没说完,他就生气了。 = Before she'd even finished speaking, he got angry. (2) Right after / immediately following: 下了课就回家。 = (I) went home right after class. 他一看就懂了。 = One look and he understood. (3) Easier / more readily than expected: 这个我一个人就能做。 = I can do this all by myself (no need for help). (4) Emphatic 'precisely' / 'just': 就是他! = That's the one! / It's HIM. 就在那里。 = It's RIGHT there. With completed actions, 就 very often pairs with sentence-final 了 to underline the change: 七点就来了 = was here by 7 already.
才 — later / harder / only after difficulty
才 signals that the action came later, with more effort, or only after some condition was met. (1) Later than expected: 他十点才来。 = He didn't come until 10. 我学了三年才学会。 = I only managed to learn it after studying for three years. (2) Only just / barely: 我才二十岁。 = I'm only twenty (don't expect more). 我才到。 = I just arrived (this very moment, not earlier). (3) Conditional consequence — 'only then': 你说了我才知道。 = Only because you told me did I know. 多吃饭才能长高。 = Only by eating more can (you) grow tall. Important: 才 in this 'only later / only then' sense usually does NOT take a final 了 — the lateness, not the completion, is the point. 他十点才来 is right; 他十点才来了 sounds wrong to most native speakers.
Side-by-side — same event, opposite speaker stance
Compare these pairs and feel the speaker's attitude flip. 他七点就来了 / 他七点才来。 Same time (7 o'clock), same verb (came). The first marvels at how early he was; the second complains he was late. — In context, the same 7 a.m. could be early (for an 8 a.m. start) or late (for a 6 a.m. start). The speaker, not the clock, picks 就 or 才. 她三岁就会读书 / 她六岁才会读书。 First says 'wow, reading at three!'; second says 'only managed to read at six (unusually late)'. 我学了一个月就学会了 / 我学了三年才学会。 Speed vs. struggle. The structural pattern is identical — only the adverb changes.
就 vs. 只 — 'only' done two different ways
Both 就 and 只 zhǐ can sometimes feel like 'only', but they're different. 只 = 'only' restricting quantity or set: 我只有一个。 = I only have one. 我只喜欢茶。 = I only like tea (not coffee, not anything else). It limits what's there. 就 = 'precisely / just / no more than that' often referring to identity or sufficiency: 我就要这个。 = This is exactly the one I want. 就一个人。 = Just one person (and that's enough / only one). 就 can replace 只 in some 'sufficiency' contexts but not in pure quantity-restricting ones — 我只有一本书 is right; 我就有一本书 sounds odd unless you mean 'I have ONE book and that's it (no need for more)'. Quick test: if the English is 'and that's all I have', use 只. If it's 'precisely this, no more no less', 就 is on the table.
Common collocations to memorize
一...就... = as soon as X, Y (preview of lesson 33): 我一回家就睡觉。 = As soon as I got home, I slept. A 就是 A = A is just A (whatever you say): 好就好。 = Fine, then. 才不...呢 = absolutely NOT (emphatic): 我才不去呢! = There's no way I'm going! 要不是...就... = if it weren't for X, Y would: 要不是你,我就找不到。 = If not for you, I wouldn't have found it. 才...呢 = a defiant / proud emphasis: 我才不怕呢! = Like I'm scared! These are the high-frequency idioms — recognizing them moves you from textbook 就/才 to natural speech.
