Connection between 〜い adjectives and 〜く verbs?


The following pairs of inflections got me thinking a little bit.

  * 〜く verb plain/te, for example: 働 **く** ・働 **い** て
  * 〜い adjective plain/nai, for example: 甘 **い** ・甘 **く** ない

Both of these suggest that there is perhaps some historical connection between
the く sound and い sound, either phonologically or semantically. Is there any
reason why い and く seem to like to "hang out" with each other like this? Maybe
these two classes of words diverged from the same class of words somehow?




> there is perhaps some historical connection between the く sound and い sound,
> either phonologically or semantically.

I think the answer from blutorange addresses this.

> Maybe these two classes of words [〜い adjectives and 〜く verbs] diverged from
> the same class of words somehow?

I'll disagree with blutorange about this part, as his answer is (I believe)
only intended to discuss the phonology, whereas the portion of your question
here is driving towards something deeper.

### Historical background for ～い adjective endings

In the modern language, these adjectives have three main endings: ～い for use
in the predicate (sentence-ending) or attributive (directly modifying a noun)
roles, ～く for adverbial, and ～かった for past tense. Notably, ～かった is actually a
contraction of adverbial ～く + あった, the past tense for classical copula ("is"
verb) あり, modern ある. So we're left with two endings, just ～い and ～く.

However, these ～い adjectives used to have three endings.

  * ～し for terminal / predicate use
  * ～き for attributive use  
↑ These two collapsed over time, with the attributive gradually replacing the
terminal, starting from around the late [Heian
period](https://en.wikipedia.org/wiki/Heian_period) (794–1185).

  * ～く for adverbial use

### Historical sound shifts

Some time during the [Kamakura
period](https://en.wikipedia.org/wiki/Kamakura_period) (1185–1333) the medial
(mid-word) //-k-// sound started disappearing in certain phonological
contexts. This happened for both the ～き and ～く adjective forms, and for the ～き
on the end of verb stems (the part before the ～ます) when followed by the
conjunctive ～て ending. This produced things like 書【か】きて → 書【か】いて, 強【つよ】き →
強【つよ】い, and 早【はや】く → 早【はや】う. From what I've read and can recall, it seems this
shift may have started in the Kansai region, and slowly gained adoption
elsewhere. This produced the modern standard Tokyo-dialect adjective ending ～い
and the well-known slightly-irregular ～て forms for verbs.

This loss of //-k-// plus a flattening of vowels also produced terms like おはよう
and おめでとう, from honorific お～ + the no-"k" adverbial forms 早【はや】う and 愛【め】でたう.

For reasons I don't yet understand, however, the loss of medial //-k-// for
adverbial forms did not gain full acceptance, and thus we still have that
//-k-// in the adverbial ～く ending for adjectives in standard Tokyo-based
Japanese. I believe the no-"k" adverbial is still in common use in Kansai, but
I must rely on others to confirm, as I have never spent time living in that
part of Japan.

Although the shift here is about a loss of medial //-k-//, in Japanese, this
is described as a shift towards the vowel value. So the き → い change is called
イ音便【おんびん】 (literally " _i_ sound passing-along", or " _i_ euphony"), while the
く → う change is called ウ音便【おんびん】. Read more about Japanese sound shifts at
[the Japanese
Wikipedia](https://ja.wikipedia.org/wiki/%E9%9F%B3%E4%BE%BF#%E9%9F%B3%E5%BD%A2%E4%B8%8A%E3%81%AE%E5%88%86%E9%A1%9E%E3%81%A8%E7%94%A8%E4%BE%8B)
and [the English
Wikipedia](https://en.wikipedia.org/wiki/Japanese_phonology#Onbin).

### Connection between 〜い adjectives and 〜く verbs?

Ultimately, the only connection appears to be the phonology. I have never read
anything suggesting that ～い adjectives as a class have anything to do
derivationally with verbs ending in ～く.

* * *

Please comment if the above does not address your question.




> there is perhaps some historical connection between the く sound and い sound,
> either phonologically or semantically.

I think the answer from blutorange addresses this.

> Maybe these two classes of words [〜い adjectives and 〜く verbs] diverged from
> the same class of words somehow?

I'll disagree with blutorange about this part, as his answer is (I believe)
only intended to discuss the phonology, whereas the portion of your question
here is driving towards something deeper.

### Historical background for ～い adjective endings

In the modern language, these adjectives have three main endings: ～い for use
in the predicate (sentence-ending) or attributive (directly modifying a noun)
roles, ～く for adverbial, and ～かった for past tense. Notably, ～かった is actually a
contraction of adverbial ～く + あった, the past tense for classical copula ("is"
verb) あり, modern ある. So we're left with two endings, just ～い and ～く.

However, these ～い adjectives used to have three endings.

  * ～し for terminal / predicate use
  * ～き for attributive use  
↑ These two collapsed over time, with the attributive gradually replacing the
terminal, starting from around the late [Heian
period](https://en.wikipedia.org/wiki/Heian_period) (794–1185).

  * ～く for adverbial use

### Historical sound shifts

Some time during the [Kamakura
period](https://en.wikipedia.org/wiki/Kamakura_period) (1185–1333) the medial
(mid-word) //-k-// sound started disappearing in certain phonological
contexts. This happened for both the ～き and ～く adjective forms, and for the ～き
on the end of verb stems (the part before the ～ます) when followed by the
conjunctive ～て ending. This produced things like 書【か】きて → 書【か】いて, 強【つよ】き →
強【つよ】い, and 早【はや】く → 早【はや】う. From what I've read and can recall, it seems this
shift may have started in the Kansai region, and slowly gained adoption
elsewhere. This produced the modern standard Tokyo-dialect adjective ending ～い
and the well-known slightly-irregular ～て forms for verbs.

This loss of //-k-// plus a flattening of vowels also produced terms like おはよう
and おめでとう, from honorific お～ + the no-"k" adverbial forms 早【はや】う and 愛【め】でたう.

For reasons I don't yet understand, however, the loss of medial //-k-// for
adverbial forms did not gain full acceptance, and thus we still have that
//-k-// in the adverbial ～く ending for adjectives in standard Tokyo-based
Japanese. I believe the no-"k" adverbial is still in common use in Kansai, but
I must rely on others to confirm, as I have never spent time living in that
part of Japan.

Although the shift here is about a loss of medial //-k-//, in Japanese, this
is described as a shift towards the vowel value. So the き → い change is called
イ音便【おんびん】 (literally " _i_ sound passing-along", or " _i_ euphony"), while the
く → う change is called ウ音便【おんびん】. Read more about Japanese sound shifts at
[the Japanese
Wikipedia](https://ja.wikipedia.org/wiki/%E9%9F%B3%E4%BE%BF#%E9%9F%B3%E5%BD%A2%E4%B8%8A%E3%81%AE%E5%88%86%E9%A1%9E%E3%81%A8%E7%94%A8%E4%BE%8B)
and [the English
Wikipedia](https://en.wikipedia.org/wiki/Japanese_phonology#Onbin).

### Connection between 〜い adjectives and 〜く verbs?

Ultimately, the only connection appears to be the phonology. I have never read
anything suggesting that ～い adjectives as a class have anything to do
derivationally with verbs ending in ～く.

* * *

Please comment if the above does not address your question.

