刚才是时间副词，如：刚才你在干什么？ 刚刚是时间和程度副词，如：我刚刚来的（时间）；这样就刚刚好了（程度）

刚刚 is closer to now than 刚才。 我刚刚回来。（it's like just a minute ago or even less than a mimute ago I arrived home) 刚才他说什么？ （刚才is a vague concept of time that is not long after something you're talking about with 刚才。 刚才 suggests a short time ago. But as in English, nobody could tell you, a short time is exactly for how long. ) I hope this can help.

刚才只可以用来形容时间 刚刚不仅可以形容时间，还可以形容程度，比如“刚刚够”。