The “一……就……” pattern
The pattern “一……就……” links two actions very closely.
It is used in two main senses:
as soon as the first action happens, the second follows at once;
the first is a condition or cause, the second its result.
It is usually rendered as:
“as soon as …, …”
“the moment …, …”
Basic structure
一 + action 1 + 就 + action 2
That is: the moment one thing happens, the other follows.
When the subject is the same
If one person does both actions, the subject may stand before 一 or before 就.
Subject + 一 + V₁ + 就 + V₂
or
一 + subject + V₁ + 就 + V₂
When the subjects differ
If the two actions have different subjects, each subject precedes its own action.
一 + subject 1 + V₁，subject 2 + 就 + V₂
Examples
你怎么一下课就往外跑？
Why do you rush outside the moment class ends?
下课class ends
往外跑run outside
一下课就……as soon as class ends, …
The second action follows immediately:
下课 → 马上往外跑
Class ends → straight outside.
我一到家，妈妈就打来电话了。
The moment I got home, Mum rang.
The two actions here have different subjects.
我一到家the moment I got home
妈妈就打来电话了Mum rang straight away
一 + 我 + 到家，妈妈 + 就 + 打来电话了
一到星期六，陈天中就跟同学去打篮球。
As soon as Saturday comes, Chen Tianzhong goes to play basketball with his classmates.
一到星期六as soon as Saturday comes
就跟同学去打篮球goes to play basketball with his classmates
Quick rule
一 + A，就 + B
Meaning: as soon as A happens, B happens.
一下课就回家。— He goes home the moment class ends.
一到家就休息。— He rests as soon as he gets home.
Remember
In “一……就……”, 一 is not the number “one”.
一…… → the first action taking place
就…… → what follows immediately
So 一到家就睡觉。is not “having come home once” but: He goes to sleep as soon as he gets home.