刚，刚刚 and 刚才 can be used to indicate time. Sometimes, they are interchangeable. But, 刚才 is used to emphasize the subject was doing something during a short period of time, while 刚/刚刚 is used to emphasize the issue took place only a moment ago.
我刚才打了一个电话。(Wǒ gāngcái dǎle yīgè diànhuà.) I just made a call.
>> Emphasize the thing
我刚刚/刚打了一个电话给她，她不会忘记的。(Wǒ gānggāng/gāng dǎle yīgè diànhuà gěi tā, tā bú huì wàngjì de.) I just made a call to her, and she will not forget it.
>> Emphasize the time
刚才 is a noun, which is used before or after the subject to indicate a short period of time before speaking.
我刚才去学校了。(Wǒ gāngcái qù xuéxiàole.) I went to school just now.
刚才我看见汉语老师了。(Gāngcái wǒ kànjiàn hànyǔ lǎoshīle.) I saw a Chinese teacher just now.
公司刚才来了很多记者，怎么了？(Gōngsī gāngcái láile hěnduō jìzhě, zěnme le?) A lot of reporters came to the company just now. What’s wrong?
刚才电话响了一下，是你打的吗？(Gāngcái diànhuà xiǎngle yīxià, shì nǐ dǎ de ma?) The phone rang just now, did you call it?
刚才 could add a noun after it.
刚才 +的 + noun
刚才的事情，很抱歉。(Gāngcái de shìqíng, hěn bàoqiàn.) I’m sorry for what happened just now.
刚才的电话是谁打的？(Gāngcái de diànhuà shì shéi dǎ de?) Who made the call just now?
When using “比” or “跟”, we can only use with 刚才.
比 / 跟 + 刚才
现在的雨比刚才还大。(Xiànzài de yǔ bǐ gāngcái hái dà.) The rain is heavier now than before.
跟刚才差不多，他还在发烧。(Gēn gāngcái chàbùduō, tā hái zài fāshāo.) Like just now, he is still feverish.
跟刚才比，现在我感觉舒服一点儿了。(Gēn gāngcái bǐ, xiànzài wǒ gǎnjué shūfú yīdiǎnr le.) Compared to before, I feel a little more comfortable now.
The negative form:
刚才 + 没/不 + verb
我刚才没看见汉语老师。(Wǒ gāngcái méi kànjiàn hànyǔ lǎoshī.) I didn’t see the Chinese teacher just now.
为什么你刚才不写作业？(Wèishénme nǐ gāngcái bù xiě zuòyè?) Why didn’t you do your homework just now?
我刚才不想睡觉，但是现在非常困。(Wǒ gāngcái bùxiǎng shuìjiào, dànshì xiànzài fēicháng kùn.) I didn’t want to sleep just now, but I am very sleepy now.
刚 or刚刚 is an adverb. They are followed by a verb or an adjective to refer to a point in time shortly before speaking.
刚/刚刚 + verb / adjective
我刚刚到家。(Wǒ gānggāng dàojiā.) I just arrived home.
同事刚刚收到我的邮件。(Tóngshì gānggāng shōu dào wǒ de yóujiàn.) My colleague just received my email.
米饭刚熟，快吃吧。(Mǐfàn gāng shú, kuài chī ba.) The rice is just cooked, eat it quickly.
When using 就 or 又 with 刚/刚刚，it indicates events take place in succession.
刚/刚刚 + 就 / 又
我刚打电话，你就来了。(Wǒ gāng dǎ diànhuà, nǐ jiù láile.) I just called, then you came.
老师刚走，你就大声说话。(Lǎoshī gāngzǒu, nǐ jiù dàshēng shuōhuà.) As soon as the teacher left, you talked loudly.
我刚刚到教室，老师又说不上课了。(Wǒ gānggāng dào jiàoshì, lǎoshī yòu shuō bú shàng kè le.) As soon as I arrived in the classroom, the teacher said that there was no class.
When using 想 or 要 with 刚/刚刚，it indicates something is about to happen.
刚/刚刚 + 想 / 要
我刚刚想离开，你就来了。(Wǒ gānggāng xiǎng líkāi, nǐ jiù láile.) I just wanted to leave when you came.
孩子刚想哭，妈妈就回来了。(Háizi gāng xiǎng kū, māma jiù huílái le.) As soon as the child wanted to cry, his mother came back.
我们刚刚要买东西，就发现钱包不见了。(Wǒmen gānggāng yāomǎi dōngxi, jiù fāxiàn qiánbāo bújiàn le.) We just found out that the wallet was missing when we were about to buy something.
刚/刚刚 can be used to indicate just reaching a certain point, just fit the requirement, just right.
衣服刚好，不大不小。(Yīfu gānghǎo, bù dà bù xiǎo.) The clothes are just right, not too big or too small.
行李刚刚二十公斤，正好。(Xínglǐ gānggāng èrshí gōngjīn, zhènghǎo.) The luggage is just twenty kilograms, which is just right.
刚/刚刚 can be used to indicate barely reaching a certain degree, only.
声音太小了，刚可以听到。(Shēngyīn tài xiǎole, gāng kěyǐ tīng dào.) The sound is too small, just can be heard.
时间刚刚够用。(Shíjiān gānggāng gòu yòng.) Just enough time.
The negative form:
不是 + 刚/刚刚 + verb
他不是刚刚离开的，他早就离开了。(Tā búshì gānggāng líkāi de, tā zǎo jiù líkāile.) He didn’t just leave, he left long ago.
饭不是刚做的，是昨天的。(Fàn búshì gāng zuò de, shì zuótiān de.) The rice was not just made, it was from yesterday.
In a sentence with刚/刚, the verb can be followed by words or expressions denoting a period of time, while 刚才 cannot use in this way.
刚/刚刚 + verb + a period of time
我刚来十分钟。（Wǒ ɡānɡ lái shí fēnzhōnɡ.）I just come here for ten minutes.
他们刚刚在这里坐了一会儿。(Tāmen ɡānɡɡānɡ zài zhèli zuò le yíhuì’r.) They just sat here for a while.
你刚跑了一个小时，累了吧？(Nǐ gāng pǎole yígè xiǎoshí, lèile ba?) You just ran for an hour. You must be tired, right?
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