ない (rōmaji -nai)
- used to form derivative -i adjectives from other terms: having that quality, having that state; very much that quality or state
- setsunai, itokenai, gikochinai
- very moving, really young of manner, having clumsiness
ない (-i inflection, rōmaji nai)
- In public speech and written language, irregular expressions ありません (arimasen) and ありませんでした (arimasen deshita) (past) are recommended instead of ないです (nai desu) and なかったです (nakatta desu).
First appears in texts from the late Muromachi period as an eastern-dialect term. Sometimes described as related to ancient eastern-dialect negative ending なふ (nafu), but there is a sizable gap of time between the apparent disappearance of nafu and the emergence of nai. Moreover, nafu conjugated as a verb, whereas nai conjugates as an adjective.
That said, both nafu and nai probably derive ultimately from ancient copula or stative verb ぬ (nu), with the negative sense possibly originating from the 未然形 (mizenkei, “irrealis or incomplete form”) of the verb stem, to which these endings attach.
The nai auxiliary conjugates as a regular -i adjective just like the adjective nai in modern Japanese, but the patterns for the auxiliary were originally different from the adjective. In the Edo period, the auxiliary conjugated irregularly, including nanda instead of modern nakatta (past), and naikereba instead of modern nakereba (conditional).
ない (rōmaji -nai)
The auxiliary nai and the adjective 無い (nai) have mostly converged in modern usage. One distinction that is still maintained by some speakers appears when attaching そう (sō, “appears like, seems like”) or すぎる (sugiru, “too, too much, in excess”):
- Adjective nai + sō or sugiru → nasasō, nasasugiru
- Auxiliary nai + sō or sugiru → nasō, nasugiru
The process of convergence is ongoing, and the さ-infix might be used by some speakers even with the auxiliary. This could be considered as proscribed in formal contexts, as slang or a grammatical error.