刚 and 刚才: just, a moment ago
副词 / adverbs (时间)
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
Subject + 刚 + Verb / 刚才 + Subject + Verb
Both refer to the very recent past, but they're different words:
刚 (adverb) = "just": goes before the verb, never at the start. Usually no 了: 我刚到公司. It can cover a longer span (我刚来北京一个月 = I only came to Beijing a month ago).
刚才 (time noun) = "a moment ago": can start the sentence or follow the subject, and takes 了 and negation: 刚才你去哪儿了？
Examples
我刚到办公室。
Wǒ gāng dào bàngōngshì.I've just arrived at the office.
刚才经理找你了。
Gāngcái jīnglǐ zhǎo nǐ le.The manager was looking for you a moment ago.
他刚来公司两个月。
Tā gāng lái gōngsī liǎng ge yuè.He only joined the company two months ago.
刚才的会你为什么没来？
Gāngcái de huì nǐ wèi shénme méi lái?Why didn't you come to the meeting just now?
Common mistakes
✗ 刚我到了 → ✓ 我刚到 / 刚才我到了.
✗ 我刚没去 → ✓ 我刚才没去 (刚 can't be negated).