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There are at the very least several i-adjectives can be used as na-adjectives
by dropping the final い and adding な in its place. The most common examples of
this, as far as I am aware, are 大きい and 小さい, which become 大きな and 小さな,
respectively. For quite a while, these were the only examples I was aware of,
and so I was able to accept them as just "an exception to the rule." However,
as I was just watching an anime, I came across おかしな, which I'd not heard
previously.

_Update._ Ignacio Vazquez-Abrams pointed out in a comment that 暖かい can also be
used in this way (i.e. 暖かな). I guess that means I have to take back my claim
that only the three above can be used as na-adjectives. I will continue to
search.

* * *

_Update 3._ It's been quite a while since I originally asked this question,
and it appears that in the interim, dainichi has found two more instances of
where this is possible. Specifically, やわらかい → やわらかな and 細【こま】かい → 細【こま】かな. So
maybe there are some criteria to determine which adjectives can be used in
this way, after all.

In the interest of getting a definitive answer, I've decided to offer a bounty
on this question. The best answer will address the following points fully,
with sources cited where possible (academic research or dictionary entries /
grammar resources are preferred):

  1. It has been suggested that the ～な usage adds a higher level of subjectivity; **is confirmation of this assertion available?**

  2. We have determined a list currently of six adjectives which can be used in this way, outlined below. **Are there any other i-adjectives which can be used as na-adjectives in this manner?**

  3. **Is there a system or set of criteria which can be used to identify adjectives which can be used in this way?**

a. If YES, **what is that system or set of criteria?**

b. If NO, **why can this set of adjectives be used in this way?**

If you would like the question to be clarified further, please leave a comment
and I will be more than willing to do so if possible.

* * *

> **Currently Identified i→na Adjectives**
>
>   * 大【おお】きい ⇒ 大【おお】きな
>   * 小【ちい】さい ⇒ 小【ちい】さな
>   * おかしい ⇒ おかしな
>   * 暖【あたた】かい ⇒ 暖【あたた】かな
>   * やわらかい ⇒ やわらかな
>   * 細【こま】かい ⇒ 細【こま】かな
>

* * *

Related question: [Why does Japanese have two kinds of adjectives? (-i
adjectives and -na
adjectives)](https://japanese.stackexchange.com/questions/1008/why-does-
japanese-have-two-kinds-of-adjectives-i-adjectives-and-na-adjectives)
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grammar nuances adjectives
i-adjectives used as na-adjectives: is there a difference? (e.g. 大きい versus 大きな)
> And finally, is there any difference in nuance or feeling when these are
> used as na-adjectives versus i-adjectives?

Short answer: Consensus is "using the な version makes the adjective more
subjective".
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The method foreigners learn Japanese and Japanese learn Japanese is different.
The concept of い形容詞 and な形容詞 doesn't exist to natives.

**Native Grammar 形容詞**

* Your い Adjectives.
* Ex: 大きいもの
* Ends in い so follow your rules you know

**Native Grammar 形容動詞**

* Your な Adjectives.
* Ex: きれいな、きれいだ、きれいに
* Has に、だ following

**Native Grammar: 連体詞**

* _Some_ are your な Adjectives. (some of your exceptions in your listing)
* Ex: 大きな、この、あの、その
* Never followed by に、だ（you never say 大きいだ）

Entire books are written on this subject alone, however, I recommend you
either (a) continue how you're learning and learn the situational uses, or (b)
spend several months studying Japanese from Japanese books themselves
(requires advanced level). The terms I listed should help you further in your
studies.

I was fortunate to have both opportunities, the latter at a Japanese
university. Japanese, like most modern languages, is a mess, really, so don't
always worry so much about the logic behind it.
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This is what I have concluded:

［大き・な, 小さ・な, [可笑]{おか}し・な］

Form a class of "[連体詞]{れんたいし}" ("adnominals") that are either な-adjectives
that are either restricted to 連体形 ("modifying form") only or pseudo
な-adjectives that only have a 連体形. However, because the restrictions of these
three are not exactly the same as other 連体詞 (like for instance この/その/あの) they
are often listed as な-adjectives, usually with a note saying that their usage
is limited.

All other 連体詞 are actually verbs/adjectives/nouns/phrases that follow
classical grammar rules that have been restricted for modern use, or rather
most aspects of their original forms have been deprecated. (いわゆる is a great
example of this, as it is 言ふ conjugated with the Heian auxiliary verb ゆ).

As for addressing the other adjectives mentioned (and also 大きい/小さい):

暖かい→暖かな  
やわらかい→やわらかな  
細かい→細かな

If you trace them etymologically they are just two separate forms of
adjectives that branched from the same source. Theoretically, there should be
no difference between them, though they may naturally have gained some
nuances. I've never heard of the "subjective" argument though.

Etymology chains for those curious:  
_many of these motions are based on not 100% accepted theory_

**あたたか・い／あたたか・な**

あつ(v)→あたつ(v)→あたた(n)→あたたか(n/n-adj)→あたたか・し(adj)

**やわらか・い／やわらか・な**

やわ(n)→やわらか(n/n-adj)→やわらか・し(adj)

**細か・い／細か・な**

こむ(v)→こま(n)→こまか(n/n-adj)→こまか・し(adj)

**大きい／大き・な**

おお(n)→おお・し(adj)→おお・き(adj's 連体形)→おおき・し(adj)→おおき(n-adj)

**小さい／小さ・な**

ちす(v)→ちさ(n)→ちいさ(sound change)→ちいさ・し(adj)→ちいさ(n-adj)

While some of the end bits of that probably occurred in a 1→2 fashion, some of
them may have evolved concurrently, so it would be hard to make a set rule or
assumption based on how the word was derived. However, if anything, it seems
clear that all of the na-adjectives that have regular adjective counterparts
that are not strange 連体詞 anomalies were na-adjectives **before** they were
adjectives. I wouldn't say the reverse is valid right away because 小さな／小さい
doesn't have any indication that 小さい came before 小さな, but 可笑し・な was clearly an
しく adjective before they it was a (pseudo) な adjective.

That's my two cents.

If we could find any more examples of those sort of dual/form adjectives to
give merit to the above assumption/theory that would be awesome, but I doubt
it'd be enough to make it substantial.
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## Some Preliminaries

I first think it's necessary to clear up something that has been confusing me
from the start: **we are actually talking about two different classes of
words.** The first is the original set of three: 大【おお】きい、小【ちい】さい、可笑【おか】しい. The
rest of the words we're talking about are all different than these three.

## The Special Three... And All The Rest

Why are these three special? To answer that, we have to take a look at the
grammar of Japanese adjectival forms, what we commonly know as
**i-adjectives** (形容詞【けいようし】) and **na-adjectives** (形容動詞【けいようどうし】). It's a
bit more complicated than that in reality, but those are the biggest two sets
and the most relevant to our discussion.

One thing that makes these three words special is that **they are i-adjectives
with a na-adjective form.** That is also true of the rest of our examples,
which I will come back to shortly. What is extra special about these three
words, _and no others_ 1 is that they are _incomplete_ na-adjectives that can
only be used to modify a noun; they are like, _broken_.

All the other words that have been suggested, and indeed all those that
_could_ be suggested, have fully-fledged na-adjective forms that can be used
in any situation a normal na-adjective can.

## The Nitty Gritty & Some Examples

Conjugable words in Japanese (用言【ようげん】, essentially verbs and adjectives) have
six different **stem forms** (活用形【かつようけい】). I'm going to talk about three of
them, because they're pretty common with both types of adjective:
**continuative form** (連用形【れんようけい】), **terminal form** (終止形【しゅうしけい】), and
**attributive form** (連体形【れんたいけい】).

With normal adjectives, we can use all of these forms:

>   * ○ ゲームを **安【やす】く** 買【か】えるお店【みせ】 (continuative form)
>   * ○ ゲームが **安【やす】い** (terminal form)
>   * ○ **安【やす】い** ゲーム (attributive form)
>

That's true of na-adjectives as well:

>   * ○ **幸【しあわ】せで** いられる人【ひと】 (continuative form)
>   * ○ **静【しず】かに** 読【よ】む (continuative form)
>   * ○ その人【ひと】が **幸【しあわ】せだ** (terminal form)
>   * ○ **幸【しあわ】せな** 人 (attributive form)
>

There are a couple other ways to make some of these forms, but these are the
prototypical examples. Now, here's the deal: the three words in the "special"
class I discussed above (大【おお】きい、小【ちい】さい、可笑【おか】しい) **can _only_ be used in the
attributive form**. So we end up with something like this:

>   * × **可笑【おか】しで** 描【か】く (bad, continuative form)
>   * × 絵【え】が **可笑【おか】しだ** (bad, terminal form)
>   * ○ **可笑【おか】しな** 絵【え】 (good, attributive form)
>

This pattern is true for all three of these adjectives, and not for any other
i-adjective. The difference is that with any of the other examples, all of the
other stem forms are possible, for example:

>   * ○ **柔【やわ】らかに** 焼【や】く
>   * ○ パンが **柔【やわ】らかだ**
>   * ○ **柔【やわ】らかな** パン
>

## Conclusions

So, let me get back to the questions at hand.

### Does the ～な usage really add a higher level of subjectivity?

Honestly, I don't know. Most of my research was on the usage and
classification, not actually on the resulting meanings. It was suggested that
the [n] sound in Japanese tends to add a softer feeling to words2, and that
may influence the meaning here.

### Are there any other i-adjectives which can be used as na-adjectives in
this manner?

Yes and no... other than our Special Three, there are no i-adjectives that can
be used as na-adjectives in only a limited capacity. There are, however, a
number of other i-adjectives that also function as na-adjectives, for example,
those [suggested in a
comment](https://japanese.stackexchange.com/q/1853/384#comment20081_1853) by
[@user1205935](https://japanese.stackexchange.com/users/1628/user1205935) (and
I imagine there are any number more):

>   * 四角【しかく】い ⇒ 四角【しかく】な
>   * 真【ま】っ白【しろ】い ⇒ 真【ま】っ白【しろ】な
>   * 真【ま】っ黒【くろ】い ⇒ 真【ま】っ黒【くろ】な
>

### Is there a system or set of criteria which can be used to identify
adjectives which can be used in this way?

Since I've divided the words up into two groups, this warrants two answers. A
research paper I found1 suggests that words which are both i-adjectives and
fully-functional na-adjectives generally fall into one of several categories,
including **color** , [edit this].

On the other hand, our Special Three seem to be unique, so there isn't much of
a system, per se. And finally...

### What makes the Special Three special?

That's a difficult one. I have yet to find a satisfactory answer; I suspect
that no one knows, exactly. There are a number of theories (for example, that
explained in [@Kafka
Fuura](https://japanese.stackexchange.com/users/433/kafka-fuura)'s
[answer](https://japanese.stackexchange.com/a/9607/384)) but there isn't
anything approaching a real consensus. Unfortunately, this one is probably
going to remain a mystery for now.

## Side Notes

Now, there is actually a class of words which encompasses our Special Three:
attributive words (連体詞【れんたいし】). There are actually quite a few of these words
that can only be used in front of a noun, and they fall into several smaller
subcategories: attributives ending in ～の (e.g. あの、この、その); attributives ending
in ～る (e.g. いわゆる); and attributives ending in ～た (e.g. たいした).

Of course, our Special Three (the only attributives ending in ～な) function a
bit differently than all of the others.

So for that reason, I don't much care for the classification of "attributive
words" -- it combines a lot of words which work in very different ways
together into one category, ignoring the underlying nuances. And the Special
Three, which are even more special.

But you should probably know that category does exist. Technically.

## References

1 Backhouse, A. E. (1984). _Have all the adjectives gone?_. Lingua, 62(3),
169-186.  
2 Makino, S., & Tsutsui, M. (1989). _A dictionary of basic Japanese grammar:
Nihongo kihon bunpō jiten (Vol. 1)_. Japan Times (Tokyo, Japan).

_Note: I can provide a copy of the Backhouse article if you're interested in
reading it._
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Very interesting question as it is mixing a few different concepts.

  * い adjective 形容詞【けいようし】 (e.g.: 大きい、小さい)
  * だ adjective-verb 形容動詞【けいようどうし】 (e.g.: 便利だ、綺麗だ)
  * 〜な (大きな) 連体詞【れんたいし】 (e.g.: 大きな、小さな)

The main confusion comes from the fact that the adjective-verb 連体形【れんたいけい】
looks like a 連体詞【れんたいし】. Consider the following examples:

> 綺麗な指輪。 A beautiful ring  
>  大きな丘。 A big hill

One could be easily tempted to say this is a similar grammatical construction,
but it is not.  
In the first sentence it is a adjective-verb (形容動詞) 連体形 form, and in the
second sentence, it is a 連体詞.

One way to know if 大きな is a 形容動詞 or not is to try to conjugate it as a 形容動詞
would, eg: 大きだった, that should feel totally unnatural.

So the previous example conjugated will be:

> 綺麗だった指輪。 (still a 形容動詞)  
>  大き **かった** 丘。 (switched to a 形容詞)

A rule of thumb in this case is that any い adjective you see with a な ending
is not used as an adjective-verb, but as a 連体詞 and thus cannot be conjugated
in the same fashion of an adjective-verb.

連体詞 are a very unique grammatical structure, and even native speakers
sometimes conceptually mix them with 形容詞 or 形容動詞。

A few links in case you want more information:

  * [link](http://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q1035619725)
  * [link2](http://katsujuku8317061.ti-da.net/e2993390.html)
  * [link3](http://ameblo.jp/kokugo-doushi/entry-10912024943.html)
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### Addendum

As others have explained, most of these are pairs of ～い and ～な adjectives that
both arose in parallel from single roots.

The oddballs are 大きな・小さな・可笑しな, which exist **only** as adnominals -- they must
be followed by nouns, and cannot be used predicatively (at the end of a
sentence) -- and which have no corresponding に adverbial forms.

**However** , one of these three is not quite as odd. 可笑【をか】し existed as the
well-known terminal form of modern adjective 可笑【おか】しい, _but_ it also existed
as the noun 可笑【をか】し・可笑【をかし】. Definitions and quotes, excerpted from
Shōgakukan's 国語大辞典 (emphasis, furigana, and dates added by me):

> **1** 笑【わら】うべきこと。滑稽【こっけい】。戯【たわむ】れ。＊史記抄【しきしょう】 (1477)‐一七「 **をかし**
> をすれども道【みち】に合【あ】ふぞ」  
>  **2**
> 猿楽【さるがく】、間【あい】狂言【きょうげん】などの滑稽【こっけい】な劇【げき】。また、それを演【えん】ずる人【ひと】。＊看聞【かんもん】御記【ぎょき】‐永享四年
> (1432) 一〇月一〇日「​ **拍子咲（ヲカシ）** ​などは男也」

This root is probably derived in turn from now-archaic verb 招【を】く as appears
in the 古事記【こじき】 and the 万葉集【まんようしゅう】, and as suggested at [Gogen-
Allguide](http://gogen-allguide.com/o/okashii.html), from the sense of
"something that causes one to draw near (because it's funny and interesting)".
I believe the な conjugation derives from the noun form.
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