enough用法問題?

這裡的enough是「代名詞」，當作動詞study的受詞。表示「足夠的量」。

請參見《新世紀英漢辭典》p.459 解釋及例句：
He has eaten quite enough.（當受詞）
Enough has been said on the problem.（當主詞）
Enough is as good as a feast.（當主詞）
He earns just enough to live on.（當受詞）
There are enough of the candies to have one each.（當主詞）

see p. 511 Collins Cobuild English Dictionary for Advanced Learners (the third edition)
"Enough" is also a pronoun.
Although the UK says efforts are being made, they are not doing enough.（當do的受詞）

In this pattern;He didn't study (hard) enough to pass the exam.
(hard) is adv.
"enough" is adj. to qualify "hard".

He didn't study(vi.) enough(adv.) to pass the exam. 華文譯：他不夠地努力以通過該考試。← enough(adv.) 修飾 study(vi.)。
enough [副詞] 釋義-1：十分に，(…するに)足りるほどに((for ...；to do)) 例. He does not work enough.
enough adv. 副詞 釋義-1：足夠地 例. the meat is not cooked enough 肉沒有熟透