i-adjectives used as na-adjectives: is there a difference? (e.g. 大きい versus 大きな)

Question

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.

So, is there a rule for what i-adjectives can be used in this way?
Alternately, if there is not a rule, per se, which ones can be used in this way?
And finally, is there any difference in nuance or feeling when these are used as na-adjectives versus i-adjectives?

Related question: Why does Japanese have two kinds of adjectives? (-i adjectives and -na adjectives)

Update: After searching through EDICT via the Japanese <-> English Dictionary Server, I was unable to find any other instances of this phenomenon aside from the three cited above, so that answers the first two parts of my question. Instead, I ask this: why can these three four i-adjectives be used in this way?

Update 2: 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. Thanks!

---

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

Answer 1

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 if 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 others¹ 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 by @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's answer) 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.}

Answer 2

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".

Answer 3

Very interesting question as it is mixing a few different concepts.

• い adjective [形容詞]{けいようし} (eg:大きい、小さい)

• だ adjective-verb [形容動詞]{けいようどうし} (eg:便利だ、綺麗だ)

• 〜な (大きな) [連体詞]{れんたいし} (eg:大きな、小さな)

The main confusion come 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 sometimes conceptually mix them with 形容詞 or 形容動詞。

A few links in case you want more information:

• link
• link2
• link3

Answer 4

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 ("modifiying 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.

Answer 5

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.