Gāng cái(刚才) and gāng(刚) are used to indicate that an action or event only took place a very short time ago. The most prominent distinction between these two words is that gāng(刚) is a temporal adverb, while gāng cái(刚才) functions as a temporal noun.
As a temporal adverb, gāng(刚) could be placed before verbs, some adjectives and numeral phrases, indicating that an action just happened or a situation just actualized a short time ago from the speaker’s point of view, which can be a few minutes, or even a few months. It is noteworthy that when using gāng(刚) with a verb, perfective aspect particle le (了) is not necessary.
wǒ gāng chī fàn.
I just started my meal.
wǒ gāng lái zhè lǐ liǎng gè yuè.
I’ve been here for only two months.
wǒ de xíng li gāng èr shí gōng jīn , méi chāo guò guī ding.
My luggage is just twenty kilograms, it doesn’t exceed the requirement.
tā gāng cóng xué xiào huí lái.
He just came back from school.
It should be noticed that the prepositional phrase normally appears before the verb phrase, the following expression is ungrammatical.
tā cóng xué xiào gāng huí lái.
Gāng cái(刚才) is a temporal noun, which indicates that the time elapsed is quite short. It can appear before verbs, adjectives the subject in the sentence. When used before a verb, gāng cái(刚才) emphasizes that something happened "just now”, which usually refers to a few minutes ago, and usually occurs with perfective le (了) as it refers to the past.
(gāng cái) wǒ (gāng cái) jiē dào le tā de diàn huà.
I just got a call from him.
wǒ gǎn jué bǐ gāng cái hǎo yī xiē le.
I feel a little better than just before.
Also, as a temporal noun, gāng cái(刚才) can directly modify nouns, while gāng(刚) cannot.
tā hǎo xiàng wàng jì le gāng cái de shì qíng.
She seems to have forgotten what happened just now.
Another distinction between gāng cái(刚才) and gāng(刚) is that, gāng cái(刚才) is used with méi (没) or bù (不)to negate the past, gāng(刚) is often used with bú shì (不是) to negate.
wǒ bú shì gāng xiě wán zuò yè, wǒ zǎo jiù xiě wán le.
I didn't just finish my homework. I finished it yesterday.
wǒ gāng cái méi zài xiě zuò yè ,wǒ zài kàn shū.
I wasn't doing my homework just now, I was reading a book.
zhè me zhòng yào de shì qíng nǐ gāng cái wéi shí me bù shuō?
It is such an important thing, why didn’t you say just now?