My dictionary says 助動詞{じょどうし} means "auxiliary verb". This sounds intuitively correct; 動詞 means verb, so 助動詞 sounds like a type of verb.

But the 助動詞「ない」 looks like an auxiliary adjective, not an auxiliary verb.

This is confusing. I suppose I don't know exactly what a 助動詞 is. Does it mean that ない is a type of verb? That seems strange, because it doesn't look much like a verb. Or is 助動詞 a wider category than just "auxiliary verbs"? That seems strange, too, because they're called 動詞.

What exactly is ない?

  • All negative form of verb/word ending with -ない can be conjugated like i-adjective.
    – oldergod
    Commented Feb 15, 2013 at 9:00

1 Answer 1


In Japanese, a 助動詞 is a conjugatable particle, as opposed to 助詞 which do not conjugate. Like noun, verb etc, 助動詞 is now considered a part of of speech. The terminology is rather unfortunate, but originally (early Meiji) it was sub-classified under the category of verb (動詞). This is due to the influence of English in which 助動詞 represents "auxiliary verbs" which express tense (will, shall, have, be), mood (will, shall, may, must, can, be), passive (be) etc.

Regarding nai, as the dictionary states, it attaches to the irrealis (未然形) form. It conjugates, thus having multiple forms: nakaro (< nakara), naku, nai, nakere. More precisely, the only form is nak-. To this, the verb ar- attaches resulting in the other forms:

  • naku + ara > nakara: To this, attaches -u and /au/ coalesce into o:.
  • naku: The adverbial / conjunctive form (連用形) as seen in all adjectives.
  • nai < naki. The attributive form as seen in all adjectives.
  • nakere: naku + are > nakere.
  • 3
    I might add that snailplane is right in that ない conjugates like an i-adjective. The unfortunate fact here is that 助動詞 means any conjugatable function words, including those which conjugate like i-adjectives and na-adjectives. It might be also considered unfortunate that 助動詞 is sometimes translated as “auxiliary verb.” Commented Dec 22, 2012 at 18:06
  • Some discussion in chat, starting from chat.stackexchange.com/transcript/message/8122088#8122088. Commented Feb 15, 2013 at 7:27

You must log in to answer this question.