1."刚”is adv.only can be used before v. 2.The meaning of “刚” is same as “刚刚”,and when you use "刚，刚刚"in sentence,you can plus a time behind the v. but "刚才"cannot be used like that. for example:1）我刚回来一会儿。but it is wrong when you say "我刚才回来一会儿“。