- It is important for you to study English.
- ⓈTo study English Ⓥis Ⓒimportant for you.
（英語を勉強することは、あなたにとって重要です...「for you」は主格補語の形容詞importantにかかる。） - ⓈFor you to study English Ⓥis Ⓒimportant.
（君が英語を勉強することが、重要なのです...「for you」は不定詞句の意味上の主語になっている。）
for 20. You use for when you make a statement about something in order to say how it affects or relates to someone, or what their attitude to it is.（主格補語にかかる場合）
- It would be excellent experience for him to travel a little.（これは「for him」を不定詞句の意味上の主語にすると文意がおかしくなる。）
- For her, books were as necessary to life as bread.
for 21. After some adjective, noun, and verb phrases, you use for to introduce the subject of the action indicated by the following infinitive verb.（不定詞句の意味上の主語）
- It might be possible for a single woman to be accepted as a foster parent.
- He held out his glass for an old waiter to refill.
「It is important for you to study English.」の場合、日本語的には20の意味のほうが一般的・汎用的に感じ、21の方だと「君が」が強調されて使用できる文脈が限られるような気がします。しかし面白いことに、ネイティブに聞くとこの文は21の不定詞句の意味上の主語として意味で取ることが多いようです。
Oxford English Dictionary
for 18.a. Governing a noun or personal pronoun followed by an infinitive, forming a construction equivalent to 'that he, etc. may, might, should', etc. Originally, the preposition had the sense A. 13 or A. 16a, the infinitive being either the subject of the sentence or expressive of purpose; but the use was early extended to include cases to which this analysis is inapplicable. In the 15-16th centuries the Latin use of the accusative and infinitive was often imitated in English: e.g. 'Behold how good..it is, brethren to dwell together in unity' (Psalm cxxxiii. 1, Prayer-bk. version).
for 13.a. Following a verb, adj., or noun of quality, denoting appointment, appropriation, fitness, etc.
- 1840 C. Thirlwall Hist. Greece VII. 283 Seleucus, reflecting on Pithon's fate, augured that which was designed for himself.
- 1840 Peter Parley's Ann. 54 What is a clock good for?
for 16.a. With the purpose or result of benefiting or gratifying; as a service to.
- 1674 A. Cremer tr. J. Scheffer Hist. Lapland 118 If he sees convenient he may set up for himself.
- 1816 Ld. Byron Parisina iii, in Siege of Corinth 65 They only for each other breathe.
元々は質や適正を表す形容詞や名詞にかかっていたfor前置詞句が、後に不定詞句の意味上の主語としても扱われるようになったようです。意味上の主語を表す「for <人> to <動詞>」は、「that <人> may/might/should <動詞>」に書き換えができます。「It is important for you to study English.」→「It is important that you should study English.（仮主語itはthat節を受けている）」
