Chinese Grammar “一……就……”

天气一热，我就想游泳。(tian qi yi re, wo jiu xiang you yong.)

In the construction “一 + Situation 1, 就 + Situation 2”, Situation 1 is the condition, whereas Situation 2 is what results directly from it.

E.g:
我一喝冰水，就肚子疼。

“一” and “就” are both adverbs and should be placed after the subject and before the predicate. In the example above, “喝冰水” and “肚子疼” are both subject-predicate construction used as predicates.

Note:
In “他一进屋就睡觉了.”, “一……就……” is not a conditional compound sentence. It indicates one action takes place immediately after another, e.g:

小王一吃完饭就出去了。