The negative form of
う-Verbs are conjugated based on the same set of rules used for other forms of
う-Verb conjugation (changing the last kana, followed by the addition of something else). In the case of negation, the additional component will be
ない (casual), or
ません (polite).
-
彼女（かのじょ）は川（かわ）で泳（およ）がない。
She does not swim in rivers.
-
彼女（かのじょ）は川（かわ）で泳（およ）ぎません。
She does not swim in rivers.
As can be seen here, the kana change differs depending on whether
ない or
ません is being used. If
ない will be used, the last kana will always change from the kana ending with a う sound, to the kana ending with an あ sound within the same row. If
ません will be used, the change will be from the う sound kana to the い sound kana. The only exception to this rule are verbs that end in the kana う. Instead of changing to あ (the あ sound kana in the same row), う will become わ, when used with
ない.
-
私（わたし）はカラオケで歌（うた）を歌（うた）あない。
I don't sing songs at karaoke. (Incorrect conjugation using あ)
-
私（わたし）はカラオケで歌（うた）を歌（うた）わない。
I don't sing songs at karaoke. (Correct conjugation using わ)
While the common translation of
ない and
ません is simply ‘not’, they actually have several possible nuances. These nuances are ‘don’t’ (usually not), ‘won’t’ (intentionally not), or ‘can’t’ (unintentionally not).
Both
ない and
ません are auxiliary verbs (ません is a conjugation of ます), meaning that they have their own set of conjugation rules.
The ない attached to make the negative form of adjectives is a different word, and therefore follows different conjugation rules. The ない attached to adjectives is an い-Adjective itself, and not an auxiliary verb.