This pattern tells us that as soon as (一, yī) one thing happened, then (就, jiù) another thing happened immediately afterward in Chinese grammar.
Structure
The pattern involves two different events, the first preceded by "一", and then the second event, which follows in quick succession, preceded by 就.
The subjects of the two "Events" can be the same, but they don't have to be. If they're the same, then you don't need to repeat the subject for the second one.
Examples
Wǒ yī kàn dào tā jiù hěn jǐn zhānɡ.
我 一 看到 他 就 很 紧张 。
我 一 看到 他 就 很 紧张 。
As soon as I saw him, I was nervous.
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