刚 / 刚才
Both 刚 and 刚才 say an action happened not long ago. Either can be translated “just”, “just now” or “a short while ago”.
你叔叔刚 / 刚才打电话来说给你发了个电子邮件
1. Position in a sentence
刚 is an adverb. It normally comes after the subject and before the verb.
Subject + 刚 + Verb
我刚看了邮箱，没有新邮件
刚才 is a time word. It can come before or after the subject.
刚才 + Subject + Verb / Subject + 刚才 + Verb
我刚才去洗手间了
刚才我去洗手间了
2. Using other time expressions
刚 can be preceded by another time expression.
Time + Subject + 刚 + Verb
我上个月刚结婚
刚才 already names a specific recent moment, so it isn't normally preceded by another time expression.
3. Duration
With 刚, the verb can be followed by an expression of duration.
他刚来一会儿
刚才 is not used this way to express duration.
4. Negative forms
刚才 can be followed by a negative such as 不 or 没.
你为什么刚才不说，现在才说？
刚 cannot normally replace 刚才 here.
Quick comparison
| 刚 | 刚才 |
|---|---|
| Adverb | Time word |
| Usually after the subject | Before or after the subject |
| Can follow another time expression | Normally cannot |
| Works with duration expressions | Not used with duration |
| Not used directly before a negative | Can be followed by 不 / 没 |
我刚吃饭了
我刚才吃饭了
刚 → focuses on the action having only just happened.
刚才 → names the recent point in time itself: “just now”.