The 一 … 就 pattern (yī … jiù) is very common in Chinese grammar. It's used to say things like “as soon as”, “once” and “whenever”, so it's a very useful and important structure to know.

The basic structure is:

[subject] 一 [action] 就 [another action]

This is equivalent to saying “As soon as [subject] does [action], they do [another action]” or “whenever [subject] does [action], they do [another action]".

You can also use this structure with two different subjects. In that case, the second subject just goes before 就. This will seem natural once you see it in the example sentences below.

Let's have a look at the slightly different meanings this structure can have one by one.

一 … 就: “as soon as”

Perhaps the most common use of the 一 … 就 pattern is to say “as soon as”. You just put the first event or action after 一, and the following event or action after 就.

Have a look at some example sentences:

我一到上海就打电话给你。

Wǒ yī dào Shànghǎi jiù dǎ diànhuà gěi nǐ.

I'll phone you as soon as I arrive in Shanghai.

小偷一看到警车就跑走了。

Xiǎotōu yī kàn dào jǐngchē jiù pǎo zǒu le.

The thief ran off as soon as he saw the police car.

他一放学就回宿舍了。

Tā yī fàngxué jiù huí sùshè le.

He went back to the dorm as soon as he finished class.

Notice how the actions are put into the same slots in each of the sentences: 一 [first action] 就 [second action].

一 … 就: “once” or “when”

Using 一 … 就 to mean “once” or “when” is actually very similar to using it to mean “as soon as”. It's just an alternative translation in English. The meaning is the same. In all cases, this pattern is about doing one thing immediately after another.

Some examples:

我工作一做完就来找你们。

Wǒ gōngzuò yī zuò wán jiù lái zhǎo nǐmen.

I'll come and find you once I've finished my work.

太阳一出来，雪就会融化。

Tàiyáng yī chūlái, xuě jiù huì rónghuà.

The snow will melt when the sun comes out.

她一戴上安全帽就感觉安全多了。

Tā yī dàishàng ānquán mào jiù gǎnjué ānquán duō le.

She felt much safer once she'd put her helmet on.

In all of the above example sentences, “as soon as”, “once” and “when” are pretty much interchangeable as translations.

一 … 就: “whenever”

Finally, this pattern can also be used to express “whenever”. This is used to talk about habitual actions that happen whenever some other event or action happens.

Let's have a look at some example sentences:

我一有问题就用谷歌查一下。

Wǒ yī yǒu wèntí jiù yòng Gǔgē chá yīxià.

Whenever I have a question, I just Google it.

我一得到一大笔钱就捐一部分给慈善机构。

Wǒ yī dédào yī dà bǐ qián jiù juān yī bùfèn gěi císhàn   jīgòu.

Whenever I get a large sum of money, I donate some of it to   charity.

那两个人一遇到彼此就开始吵架。

Nà liǎng gèrén yī yù dào bǐcǐ jiù kāishǐ chǎojià.

Those two people start arguing whenever they meet.

Hopefully you can see the common theme for using 一 … 就. It's always about things that happen right after or because of other events.