How to use 才？

才 "It means things happened late, slow or not going well. It can be used with the contrast就, to strengthen the memory."就 means that the action happens early.
“才”表示事情发生得晚、慢或进行得不顺利。可以与就对照使用练习，加强记忆。就”表示动作发生得早。

1 八点我就到图书馆了。 他才来。
2 他今天8点半才来上班。
3 我今天6点就起床了。妹妹睡到九点才起床。

才 means something happens later than you expect or something that you need to put your effort to complete.
1. 会议结束了，他才到。-- happened later than you expect.
2. 我办了三个小时，才能办完。 -- need your effort to get it done.

才 七点就起来了（early）
七点 才 起来（late）
才 吃了两个包子
才 两个人 （few）

才 + time -- it mean the sth happen late
time + 才-- it mean the sth happen early
才 + quantifier it mean small quantity