grammar - Adjectives functioning both as イ- and ナ-adjective

This question has made me think about the class of adjectives, which can function both as イ- and as ナ-adjective, e.g.

大きい   大きな
小さい   小さな
真っ白い 真っ白な (etc.)
細かい   細かな
暖かい   暖かな
四角い   四角な (etc.)

柔らかい 柔らかな

In my (non-)answer to the question, I mentioned that the difference of やわらかい and やわらかな is very subtle at best.

I am wondering, can this maybe explained in more generality? What are the differences in the word pairs above?

Answer

After some research, there seems to be little difference in meaning. In some situations, maybe ease of pronunciation is more of a guide than nuance of meaning. For example, in the Balanced Corpus of Contemporary Written Japanese (BCCWJ, 少納言), we have

大きな木  154 results
大きい木   12 results

(But 木 is clearly no abstract concept.)

However, my question stated that 大きい etc. can be used as both イ- and ナ-adjective, which seems to be only half of the truth as 大き shouldn't be considered a ナ-adjective, even though it can modify nouns as 大きな.

For example, 大き cannot be inflected, to form an adverb (cf. 静か → 静かに.)

Rather 大きな belongs to the class of [連体詞]{れんたいし} (Rikaichan calls this pre-noun adjectival or adnominal adjective, a better name would be adnoun).

連体詞 are words which cannot inflect and must modify a noun (or pronoun), e.g.

この、その、あの、……

おかしな、大きな、小さな、……

as in

このやり方は一番よい。

こうするのが一番よい。

or

空におかしな形の雲が浮かんでいる。

空に変な形の雲が浮かんでいる。

These 連体詞 seem to come in 4-5 varieties (depending on the book you read):

「～の」の形・・・この、その、あの、どの、ほんの

「～な」の形・・・大きな、小さな、おかしな、いろんな

「～た、～だ」の形・・・たいした、たった、とんだ、ばかげた

「～る」の形・・・ある、あらゆる、いわゆる、さる、きたる、いかなる、堂々たる

「～が」の形・・・わが

All this information is a summary of this site. I hope someone will find it useful.

This should also shed some grammatical light on this and this question.

In an after-thought as to why 小さい時 cannot be 小さな時 is that 小さな is a 連体詞 and must modify a (pro)noun and thus forces the meaning small time, rather than allowing for the interpretation as a sentence in which the (omitted) subject was small, which is usually translated with a relative clause, i.e. the time, when I was small or just when I was small.

So, the general rule seems to be that one can choose either one of the word pair, but when choosing the one ending in な, one chooses to modify directly the noun which must follow it and disallows constructions with relative clauses.

E.g. compare

山の大きい街 vs. 山の大きな街.

The former can mean a city with a big mountain, whereas the latter is necessarily a big city situated on a mountain. (One can probably come up with better examples.)