In order to create the negative past form of
い-Adjectives, we will need to use the conjugation form. The conjugation form of an
い-Adjective is simply created by removing the い, and replacing it with く. く is then followed by
ない (conjugated into its past form
なかった), or
あります (conjugated into its past negative form
ありませんでした).
The polite negative (past) form requires use of the
う - Verb,
ある. This is not a unique rule for the negative past, but actually the correct way that all く-form conjugations of い-Adjectives should be constructed. However, in modern Japanese, it is not considered necessary. Instead, the semi-polite form is often used.
The semi-polite form is constructed by using
なかった, and then simply adding
です.