I don't know him ___ to borrow money from him. (A)well enough (B)enough well 答案是A,那B為何不呢？

enough在當副詞用時，置於其所修飾的「形容詞」或「副詞」的「後面」。
It is good enough for me.＝它對我來說夠好了。
She paints well enough.=她畫得還可以。
see p.461《新世紀英漢辭典》

I don't know him well enough ....這裡的well是「副詞」，所以enough要放它的後面。

Grammar for word" enough:"-
"enough" is an adverb;It comes after adjectives and adverbs."well" is an adverb;
eg:-It is good enough for my job.
eg:-She's not old enough for the job.
eg:-There are money enough for all TW people.
eg:-There'll be time enough for that distribution.
"well "adv. comparative better, superlative best;
For emphasizing something, expressing double:-
eg:-I'm well aware of the problem of borrowing money from him involoved. It is not well worth a trial.

If enough is placed behind adverb ;then:-
I don't know him (A) well enough----- to borrow money from him.