one situation: If it rains tomorrow,I will stay at home. will means 就 in Chinese.如果明天下雨，我就呆在家里。ru guo ming tian xia yu, wo jiu dai zai jia li.

in the nature of things, something will happen, now can use "就"

and on the other hand, you want to reject something, you can also use "就", to express your opposite attitude.

eg. 请等我一下，我马上就来。

eg.你要我来我就来呀？我就不来！

我一见到你 我就很开心
im sooo happy when i see you.

there's no words in english what means 就 in chinese.
its only a word between somebody (or something) and something
no meaning of 就　
^_^

FYI
1. 那就这样定了。
That's settled then.
2. 你只要问他，他就会告诉你。
You have only to ask and he'll tell you.
3. 要是我再有一次机会就好了。
If only I had another chance.
4. 我马上就来。
I'll be right with you.