The two characters 就 and 才 can be very confusing to Chinese learners, and they are used quite often.
Let’s take a look at how to use them.
In the simplest usage,
就 indicates ‘earlyness’ – meaning is something like ‘already?’ or ‘this early?’
and
才 indicates ‘lateness’ – meaning is something like ‘only just now’
你怎么现在就到了?
Nǐ zěnme xiànzài jiù lái le?
Why have you arrived now?
(you’re so early, why so early)
你怎么现在才到了?
Nǐ zěnme xiànzài cái lái le?
Why have you arrived now?
(you’re so late, why are you here so late?)
我花了两天就完成了项目
Wǒ huāle liǎng tiān jiù wánchéngle xiàngmù
It took me 2 days to complete the project
(that was quite early)
我花了两年才完成了项目?
Wǒ huāle liǎng nián cái wánchéngle xiàngmù
It took me 2 years to complete the project
(that was quite late)
Both 就 and 才 can be used to emphasise a small quantity of something.
这么多人，你就点了两个菜？
Zhème duō rén, nǐ jiù diǎnle liǎng gè cài?
[There are] so many people, so why have you only ordered 2 dishes?
(why just/only 2?)
这么多人，你才点了两个菜？
Zhème duō rén, nǐ cái diǎnle liǎng gè cài?
One other major use of 才 to know about
才 is often used in 刚才 to mean ‘just’ or ‘just now’
我刚才到了
Wǒ gāngcái dàole
I just arrived.
我刚才看到你了
Wǒ gāngcái kàn dào nǐle
I just saw you.
Other uses of 就 (this character is a bit annoying)
就 can be used to emphasise a verb
我就喜欢！
Wǒ jiù xǐhuān
I just like it. (The slogan for McDonald’s in China)
我就要这个。
Wǒ jiù yào zhège
I just want this one. (the emphasis is on how much you want it)
就是 is used for emphasis, or it can mean ‘that’s exactly the case’ or ‘exactly is’
你就是我想找的人
Nǐ jiùshì wǒ xiǎng zhǎo de rén
You are exactly the person I’m looking for.
她就是张女士。
Tā jiùshì zhāng nǚshì
She is Ms. Zhang (it’s her).
就 is sometimes used in a slangy way when you don’t care about the outcome of something.
去就去吧。
Qù jiù qù ba.
Go if you want. (I don’t care)
就 is used in 就要 （when something is about to happen)
我很快就要走了
Wǒ hěn kuài jiùyào zǒule
I have to leave very soon.
我6月就要毕业了。
Wǒ liù yuè jiùyào bìyèle.
I am about to graduate, in June.
You will often see 就 in ‘if’ sentences, with 如果，要是，假如 （if）when it means ‘then’
如果你喜欢，就买吧
Rúguǒ nǐ xǐhuān, jiù mǎi ba
If you like it, then buy it.
你要是不想去，就不去。
Nǐ yàoshi bùxiǎng qù, jiù bù qù.
If you don’t want to go then don’t go.
就 also means ‘then’ in sentences like this:
你想来就来，不用提前跟我说
Nǐ xiǎnglái jiù lái, bùyòng tíqián gēn wǒ shuō
If you want to come, then come, you don’t need to tell me in advance.
你想吃什么就吃什么。
Nǐ xiǎng chī shénme jiù chī shénme.
Eat whatever you want.
（you want to eat what, jiu eat what）
The construction 一… 就 means ‘as soon as’
你一到了就给我打电话
Nǐ xiǎnglái jiù lái, bùyòng tíqián gēn wǒ shuō
As soon as you arrive, call me.
他一说，我就开始笑了
Tā yī shuō, wǒ jiù kāishǐ xiàole
As soon as he said it, I started laughing.
And 就 is also used in other grammatical patterns to indicate the result of something
只要你爱我，我就不怕了
Zhǐyào nǐ ài wǒ, wǒ jiù bù pà le
As long as you love me, I won’t be afraid.
一旦找到了工作，就有钱了
Yīdàn zhǎodàole gōngzuò, jiù yǒu qián le
Once I get a job, I’ll have money.