就 is a super important adverb in Chinese. It has multiple meanings, which are all very often used in both written and spoken Chinese.
就 as soon as, right after, etc
孩子们一放学就回家。Hái zi men yī fàng xué jiù huí jiā. Children return home as soon as school is over.
他一毕业就开始工作了。Tā yī bì yè jiù kāi shǐ gōng zuò le。 He begun to work as soon as he graduated.
就 at once, right away, very soon, be about to…
火车就要开了。Huǒchē jiù yào kāile。 The train is just about to leave.
电影马上就开始了。Diàn yǐng mǎ shàng jiù kāi shǐ le。The film is about to start.
我就来。Wǒ jiù lái。 I will be here right away.
饭就好。Fàn jiù hǎo。The meal is getting ready.
他们的视力用激光只要几分钟就可矫正。tā men de shì lì yòng jī guāng zhǐ yào jǐ fēn zhōng jiù kě jiǎo zhèng 。Their eyesight can be corrected in just a few minutes by the use of a laser.
就 as early as; already; long since
他早在1980年就来中国了。Tā zǎo zài 1980 nián jiù lái zhōng guó le。He came to China as early as 1980.
她早就将那笔钱勾销了。Tā zǎo jiù jiāng nà bǐ qián gōu xiāo le。She had long since written off the money.
就 as early as; already; long since
明天要是下雨，我们就不去了。Míng tiān yào shi xià yǔ, wǒmen jiù bù qù le. If it rains tomorrow, we won’t go.
他要是不来，我就去找他。Tā yào shi bù lái, wǒ jiù qù zhǎo tā. If he does not come, I will go to find him.
只要虚心，就会进步。Zhǐ yào xū xīn, jiù huì jìn bù。Provided you are modest, you’ll surely make progress.
只要用户满意，产品就会成功。Zhǐyào yònghù mǎnyì, chǎnpǐn jiù huì chénggōng. Make people happy, and your products will be a success.
就 comparatively a larger number, more times, stronger ability, etc.
光这一项开支就节约5万元。Guāng zhè yī xiàng kāi zhī jiù jié yuē 5 wàn yuán. So far as the expenses are concerned, we saved as much as 50,000 yuan.
他三天才来一次，你一天就来三次。Tā sān tiān cái lái yīcì, nǐ yīt iān jiù lái sān cì。He comes here once in three days but you as many as three times in just one day.