Just as 才 (cái) can express lateness, 就 (jiù) can be used to indicate that something happened earlier or sooner than expected. It can also be used in the near future to indicate something will happen very soon.
Used as "Right Away" (in the Future)
When something happens "right away," you're talking about "very soon" in the future.
The pattern is as follows:
Subj. + Time + 就 + Verb
Wǒ mǎshàng jiù lái.
I'll be there in a second.
Mǐfàn èrshí fēnzhōng jiù hǎo.
The rice will be ready in 20 minutes.
Nǐmen xiànzài jiù chūmén ma?
Are you leaving the house right now?
Tāmen yīhuìr jiù dào.
They will be here in a few minutes.
Lǎobǎn míngtiān jiù huílái.
The boss will be back tomorrow.
Note that it can sometimes be hard to translate the feeling of "soonness" into English, but in every one of these examples, the time given in the sentences feels "soon" to the speaker.
Used as "Early" (in the Past)
This use of 就 might be translated "as early as," but usually the earliness is not specifically marked in English.
Subj. + [Point in Time] + 就 + Verb + 了
Wǒmen jiǔ diǎn shàngkè, tā bā diǎn jiù lái le.
We have class at nine, but he came in at eight.
Fēijī shí diǎn qǐfēi, tā liù diǎn jiù dào jīchǎng le.
The plane takes off at ten o'clock, but he arrived at the airport at six.
Wǒ zuówǎn bā diǎn bàn jiù shuìjiào le.
I went to bed at eight thirty last night.
Tā shíbā suì jiù dàxué bìyè le.
She graduated from college when she was only 18 years old.
Not only can 就 emphasize a "point in time," but it can also emphasize a "time period," indicating that something happened very quickly.
Subj. + Time Period + 就 + Verb + 了
A few examples:
Nǐ yī gè wǎnshang jiù kàn wán le?
It only took you just one night to finish reading it?
Tā shí fēnzhōng jiù zuò wán le.
It only took him ten minutes to finish doing it.
From the example sentences it is clear that 了 naturally occurs with a verb used after 就. This is because verbs following 就 generally have the feeling of being completed.
Colloquial Saying 早就
早就 means "long ago," and is usually used to express a kind of impatience or surprise on the part of the speaker. It comes before the verb.
Wǒ zǎo jiù zhīdào le!
I knew that long ago!
Tā zǎo jiù jiéhūn le.
She got married a long time ago.
Tāmen zǎo jiù fēnshǒu le.
They broke up a long time ago.
Wǒmen zǎo jiù bìyè le.
We graduated a long time ago!
Wǒ zǎo jiù gēn nǐ shuō guo, tā bù shì hǎo rén.
I told you a long time ago that he's not a good guy.
[adapted from AllSet Learning Chinese Grammar Wiki, Creative Commons License BY-NC-SA 3.0]
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