How does 十分/不十分 work？ 十分 is an adverb used just as 非常, but while 十分 can be negated (不十分), 非常 can't. (不非常 is unacceptable.)
That's the explanation given by my book, which also gives me these sentences as examples:

他不十分聪明。
他十分不满。
他不十分满 --&gt; NO他不十分满意 --&gt; OK

Why is this last sentence (他不十分满) wrong, if the book said I can negate 十分？And why do I need to say 满意 to use 不十分?
Thank you!
Good question.

不满 is a single word. It means unhappy or unsatisfied. It cannot be separated into 不 + 十分 + 满. It's like you can't say "un-very-happy".

不满意 is a phrase, 不 + 满意 = not satisfied. It can be modified by 十分: 不 ＋ 十分 + 满意。