na-Adjective Inflection

In Japanese na-Adjectives are called 形容動詞けいようどうし (lit. "adjectival verbs"). This is an unfortunate name, because they behave like adjectival nouns + copular verb だ (and a vestige of the classical copula なり).

Inflectional Tails
Form	Inflectional Tail
未然形	ーだろ
連用形	①	ーで
	①	ーに
	②	ーだっ
終止形	ーだ
連体形	ーな
仮定形	ーなら
命令形
語幹	ー〇

Note: "ー" represents the word stem

i- and na-Adjective Comparisons

The majority of 形容詞 are of Japanese origin and 形容動詞 are created from loanwords imported from China, Korea, etc., since atleast the Heian period. The following is a comparison of similarities of the inflectional behavior of the two word classes.

i-Adjectives 形容詞	na-Adjectives 形容動詞
Adjectival verb	Adjectival noun + copular verb
Tails begin with k or i sounds	Tails begin with d or n sounds
Both have an ーアろ tail and ーアっ tail in the 未然形 and 連用形, respectively
Idiomatic expressions that use adjectives can use both i- and na-adjectives.
Neither has an imperative form (命令形)

Notes

未然形

Similar to 形容詞, the 未然形 inflection tail for 形容動詞 only has one affix: the non-inflecting suppositional auxiliary verb う.

「静かだ」→「静かだろう」

連用形

The continuative form, like many other 用言 is most productive. 形容動詞 initially seem the most complex, as they have three regularly used inflectional tails. The three tails accept affixes similar to 形容詞.

連用形 Applications
Tail	Affixes	Notes
ーで＋	① ない, ある	attaches to the auxiliary adjective ない or auxiliary verb ある
	② は・も	attaches to emphasis particles は and も
	③（中止法）	used as a conjunctive
ーに＋	用言	except the auxiliary adjectives ない and ある used with ーで all other 用言 attach to に, e.g.＋なる
ーだっ＋	た, たら, たり	only used for the three た affixes

One contrast between adjectives and verbs is the negative form or 否定形ひていけい. The inflecting affix ない is used for both verbs and adjectives, but in different inflectional forms: the negative form of verbs is created in the 未然形, whereas adjectives in the 連用形. One other trick: ない used with verbs is an auxiliary verb, whereas ない used with adjectives is actually an auxiliary adjective!

終止形

As with the other 用言, the 終止形 is sentence final – words in this form do not take additional affixes. The 終止形 inflectional tail だ makes the word complete as a predicate.

連体形

There is only one prenominal form 形容動詞 inflectional tail（な）and it only attaches to nouns. This form is what Westerners generally think of as dictionary form.

Watch out! If the 形容動詞 attaches to a noun it should be a な, but if it's a verb or adjective it should be に（連用形）

仮定形

The conditional form tail なら only takes the conditional particle ば as an affix, but ば can be omitted.

命令形

Neither 形容詞 nor 形容動詞 have an imperative form (命令形), but imperatives can still be created through the imperative form (命令形) of する. This is attached to the 連用形 of the 形容動詞 . For example:

しずかだ（終止形）→　しずかに（連用形）しろ（命令形）！

語幹のみ

The 語幹 (stem) does not constitute another formal 活用形. The 活用形 system demands an inflectional tail in each inflection form. Thus this noun-like "stem only" constitutes a special case among inflectional words and is simply treated as "outside the table."

The stem of na-adjectives is be used in several constructions where the an inflectional tail is omitted, e.g. ＋です・そうだ・らしい and simply 言い切り roles.

「静かな」→「静かです」・「静かそうだ」・「静からしい」

It is because of this stem-only useage that 形容動詞 can take the inflecting auxiliary verb です.