You asked a good question but 就 is really hard in Chinese Grammar. Here are some common ways of using it. 1.Emphasize Subj. + 就是+Noun→這是王先生嗎? 對！這就是王先生！ 2.express earliness Subj. + Time + 就 + Verb + Obj.＋了→他 下午 三點 就 吃 晚饭 了。 3.express small quantity Subj. + 就 + (＋Verb) ＋ Measure Word ＋ Noun→我們有十個人，這裡就只有三張椅子? There is no professional Chinese teacher will teach you all the meanings about 就, because it is too complex.

According to 现代汉语词典， 就 has 18 different meanings... common meanings: 1. V. 开始进入或从事（某种事业）。start to begin (a career ) 就业 就职 2.adv. 表示将在很短的时间以内发生 about to happen 我这就走 3.adv 表示前后两件事紧接着发生 A occurs immediately after B 一说话就脸红 4.v 依从. obey 半推半就

[emoji][emoji]"就" likely make your tone to be strong. For example: 1) Definitly this is you. 這就是你。 2) Hand wash before you use, is this right ? 使用之前要手洗，這不就是嗎？ [emoji][emoji]"就" can be used with another Chinese word of different meaning. But it doesn't match / fit all words. I suggest you should check the Chinese dictionary. It's likely phrasal words. For example : 1) Even if 就算 2) go bed sleep 就寢 Etc......

它是用来做什么的？