lol, my answer is too long and beyond italki's word limits, so I'll divide it into several parts and write the rest of them in my comment.
就 is a key auxiliary word used very commonly in daily conversations, it is both very significant and complex for learners. Accurate and agile use of 就 can make your sentences much more natural. Here I summed up its usages for you from a professional dictionary, and in company I'll make some sample sentences.
1. used as a preposition
1) according to, from. Often used in the form 就...(来)看 E.g. 就他所说的话来看 - ACCORDING TO/FROM what he says, ...
2) a modal particle, means "just","simply", similar to 偏偏 and 非要. E.g. 1.你不让我做，我就要做！ - You asked me not to do it, I will SIMPLY/JUST do it! 2.我就是想学钢琴，哪怕父母不支持。 - I SIMPLY/JUST want to learn piano, even my parents don't support me.
2. used as an adverb
1) right away, right now, coming soon. 立刻、马上 E.g. 火车就要来了 - The train will be arriving SOON/RIGHT AWAY.
2) often used in structure 一...就, means "as soon as". E.g. 他一放学就去图书馆了。 - He went to the library AS SOON AS he left school. It can also be used independently, indicates that after something happened, another thing will happen in a very short time. E.g 别着急，老师回答完他的问题就会来回答你的。 - Don't worry, when the teacher has answered his question, he will answer yours immediately.
3) similar to some usages of the word 'just'. E.g.1.老师就在后面，别讲话了！ - The teacher is JUST/RIGHT behind us, stop talking! (here it emphasizes the truth is just like this) 2. 雨今天早上就停了。 - The rain JUST stopped this morning. (here it is similar to 'as early as') 3.就在这个时候，... - JUST at this moment, ...
4) only,just 只、仅仅. E.g. 这里就剩下你一个吗？ - Is here ONLY/JUST you left?
5) used in front of a verb, indicates that without something, there can be no... E.g. 不学习就无法取得进步. - Without study, you can gain no progress.