- It is kind of you to help me.
- You are kind enough to help me.
しかし、意味的にはそれで近似になるとは言え、元の文におけるitはyouを受けているわけではなく、何か腑に落ちません。「It is important for you to study English.」の書き換え文「To study English is important for you.」または「For you to study English is important.」の場合、主語がそのまま置き換わっているだけですから基本的な文構造に変化はありませんが（不定詞句も名詞的用法のままです）、「You are kind enough to help me.」は文構造が大きく変化してしまっていますから、意味的にはともかくも文構造的に完全に同値とは思えません（「繰り上げ（raising）」のような何か特殊な現象が介在していれば別ですが）。
so that構文の「This book is so difficult that you can't read it.」は「Because it is very difficult, you can't read this book.」とだいたい同じような意味ではありますが厳密には異なりますし、否定文になった場合は完全に意味が食い違ってしまいますが（「This book isn't so difficult that you can't read it.」≠「Because it isn't very difficult, you can't read this book.」）、これと同じような違いが上のit＋ofの書き換えにもあるような気がします。
ネイティブに聞くと、後者の「判断の根拠の不定詞文」における形容詞が主語の人の一般的性質（「私のような人間まで助けてくれるような人なら、きっと優しい人物に違いない」）を言うものであるのに対し、仮主語it＋of構文で用いられる形容詞はその一回限りの行為に関しての評価（「助けてくれるなんて、親切だなぁ」）として捉えられるようです。
it is kind/stupid/careless etc of somebody (to do something): used to say that someone's action shows a particular quality
- It was kind of you to remember my birthday.
You use of before a word referring to the person who performed an action when saying what you think about the action.
- I suppose it's stupid of us not to be able to make up our own minds.
- This has been so nice, so terribly kind of you.
- That's certainly very generous of you Tony.
of前置詞句に入る名詞はその後に続く不定詞句内の動詞の行為者を表すとされてはいますが、やはり主格補語の後に来る形容詞が叙述するのは、その行為者ではなく行為（action）の方なようです。「優しい（kind）」のはあくまでその一回の「help me（私を助ける）」という行為であり、直接「あなた（You）」がそうだというわけではないようです。
Oxford English Dictionary
Indicating the doer of something characterized by an adjective: following an adjective alone, as foolish, good, rude, stupid, unkind, wise, wrong (or any other adjective with which conduct can be described); following an adjective qualifying a noun, as a cruel act, a cunning trick, a kind deed, an odd thing; following a past participle qualified by an adverb, as cleverly managed, ill conceived, well done. Usually followed by to do (something), as in it was kind of you (i.e. a kind act or thing done by you) to help him etc., and less frequently by that, both constructions introducing the logical subject or object of the statement, e.g. It was kind of him to tell me = His telling me was a thing kindly done by him.
「[I]t was kind of you to help him.」は「[I]t was a kind act or thing done by you to help him.」の意味で、「It was kind of him to tell me.」は「His telling me was a thing kindly done by him.」の意味になる。直接的に不定詞句の意味上の主語になる「for <人>」に対し、「of <人>」は主格補語の形容詞や名詞の方に強くつながっているようです。これは直訳すると「（あなたの）私を助けるという行為」（主語）は「あなたの優しい行為」だ（述語）といった感じになります（最初の「あなたの〜」は間接的）。日本語の文としては相当不自然になるので、これはさすがに「あなたは優しい...（You are kind (enough)....）」と意訳したくなる気持ちもわからなくはありません。
ＯＥＤの古い用例を見ると、元々は「a <形容詞> thing of <人>」だったものが「<形容詞> of <人>」へと短縮されていったようです。
Oxford English Dictionary
- 1532 W. Tyndale Expos. & Notes 73 Is it not a blind thing of the world that either they will do no good works,..or will..have the glory themselves?
- a1593 C. Marlowe Jew of Malta (1633) iv. v 'Tis a strange thing of that Iew, he lives upon pickled grasshoppers.
- 1603 W. Shakespeare Hamlet iii. ii. 101 It was a brute parte of him, To kill so capitall a calfe.
- 1668 H. More Divine Dialogues ii. 383 That's a very odd thing of the men of Arcladam.
- 1733 J. Tull Horse-hoing Husbandry 266 Is it not very unfair of Equivocus to represent [etc.]?
- 1766 H. Brooke Fool of Quality I. iv. 145 Indeed, it was very naughty of him.
- 1816 W. Scott Antiquary II. xviii. 239 Juno's master is in such fear of your joking him about the seal..--it's very silly of him, to be sure.
- 1849 F. W. Newman Soul 104 It was not a proud thing of Paul to say, but a simple truth.
- 1887 'L. Carroll' Game of Logic iv. 92 It was most absurd of you to offer it!
- 1915 W. S. Maugham Of Human Bondage lxxxvii. 456 She won't think it rude of you to sit still while she waits upon you.
- 1946 P. Larkin Let. 6 Dec. in Sel. Lett. (1992) 131 It was good of you to write so quickly in answer to my mingy letter.
- 1966 Listener 9 June 840/2 It seems to me niggling of Graham Hough to complain about the neglect of politics.
- 2000 T. Robbins Fierce Invalids 227 Kind of you to say that, Mr. Switters."
ここまで書いて一応ＯＥＤの上の説明文でGoogle検索かけてみたら、少し前に同じ問題を扱っているブログ記事が投稿されていたようです。他に『英語のしくみが見える英文法』という本にも同様の記述があるようです。
