Are there inflections/endings that can be applied to verbs but not i-adjectives? (or vice versa)


After reading in an answer to another question that Japanese adjectives are
less inflected than Japanese verbs I'm wondering if there are inflections that
can be applied to verbs but not i-adjectives? Or what about the converse?




Most of the verb endings cannot be applied to adjectives.

There are no modern potential, passive, causative, or imperative suffixes:

> x 赤られる (could be red)  
>  x 赤られる (was redded?)  
>  x 赤させる (was made red)  
>  △ 赤かれ (be red! [archaic])

Also, politeness of adjectives is encoded by the copula, and not by polite
verb endings:

> o 赤いです (is red [polite])  
>  x 赤ます (is red [polite])  
>  o 赤くありません (is not red [polite])  
>  x 赤ません (is not red [polite])

In fact, the only endings that adjectives can really have are tense, negation,
and the conditional mood.

> o 赤い (is red)  
>  o 赤かった (was red)  
>  o 赤くない (is not red)  
>  o 赤くなかった (was not red)  
>  o 赤かったら (if it were red)  
>  o 赤ければ (if it were red)




I want to add a few extra notes to Amanda's answer:

There are two different vector we should consider when comparing the coverage
of i-adjective conjugation to the verb conjugation (which is obviously
richer):

  1. **Possible inflectional bases.** Only verbs have the following bases:

    * _Mizenkei_ (未然形), a.k.a A-forms, which are used for negation: 書かない。
    * Arguably the same can be said for _renyoukei_ 連用形, a.k.a I-form, but the く form is historically related to it and fills some if its roles.

The rest of the bases are also available for adjectives. This includes the TE-
form (赤くて) suppositional form (赤かろう), and - yes - the imperative form: 赤かれ.
It's quite archaic and not very common, but you do see it a few times, so it's
good to know.

  2. **Possible _actual_ categories (= grammatical meanings) that can be expressed:**

    * For instance, while adjectives in Modern Japanese don't have a mizenkei form, they're certainly able to express negation by combining their ～く form with ない.
    * They're also definitely able to express conditions by using a slightly different base than verbs (the same base with -k- that is used for the く forms): 赤かったら and 赤ければ are both correct and rather common (they're definitely not archaic like the imperative).
    * The same goes for suppositional forms, though the first usage is also slightly archaic: 書こう, 赤かろう or simply 赤いだろう.
    * Politeness is perfectly expressible by adjectives, but just as Amanda has noted, it relies on the copula. 

The interesting categories that are not expressible by verbs are actually
these, as I see it:

  * Voice (Passive/Causative). The first one is to be expected, since i-adjectives are really stative verbs and being in a state is already an intransitive action that really is quite passive and you can't passivize it further. You can cause someone or something to enter such a state, however, so we would expect a causative form. But as it happens, such causative forms do exist, but they are irregular. You can always try to add する to the ～く form, but there are also irregularily derived verbs like 赤める and 広がる.

  * Volition. You can't describe a will to be ADJ like you could do with verbs with the ～たい form.

  * Progressive/durative aspect. A state is inherently durative, so you can't make it progressive by using ～ている forms. 赤くている is just nonsensical.

  * Many derivations using auxiliary verbs such as ～だす, ～しまう. Some other derivations only work when the state is verbalized into a process using なる, so you could say 赤くなってきた, although 赤くてきた doesn't sound natural.

