Grammar: 剛 vs 剛剛

This is a common mistake for foreign learners.

"我剛剛學這個生詞”

The student was trying to say, "I just learned the vocab."

Well, it seems correct to most foreign learners because if you look up in a dictionary you will find the translation for 剛剛 is "just".

So why is the sentence incorrect?

First of all, whenever you want to express the idea "just" in Chinese, always be careful, there are probably more than 4 different adverbs in Chinese for just, and, unfortunately you cannot use them freely.

Let's then take a closer look into these two words: 剛剛 and 剛.

1）剛剛：
It's a time word, which means it's a noun, referring to the time not too long ago from the time you speak, could be a few seconds ago or up to a few hours ago, depends on the context.

Grammatically speaking, you can use this word just like the way you use 昨天.

For example:
a. 剛剛我看到我的朋友
    我剛剛看到我的朋友
(you can put it either before the subject or right after the subject)

b. 剛剛的那個人是誰？
(you can use it alone in a 的 phrase. In this case "that person we met just then")

2) 剛：
It's an adverb, which means you can only put it right in front of a verb, referring to a feeling of lateness. Since this word is actually expressing the feeling of the speaker, it does not really put emphasis on the actual time. As a result, we can use it not only for the past but the present as well.

For example:
a. 我剛喝完一杯咖啡
    *剛我喝完一杯咖啡（x)
(must be placed right in front of the verb)

b. 每年剛開學的時候，校園總是特別熱鬧
    每年剛剛開學的時候……（x)
(剛 does not necessarily have to be the past while the time word 剛剛 must be the past.)

So let's go back to the original sentence:
我剛剛學這個生詞
The student was actually trying to say that: he just learned the vocab that morning and he got the chance to really use that word in the evening.
In this case, he's trying to express the lateness of the learning (and the earliness of using it), so the subjective adverb 剛 is the right word to use here.