Grammar JLPT Level N5 - Lesson 12

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).

• 私(わたし)は太(ふと)くなかったよ！
  I was not fat!
• 北海道(ほっかいどう)は暑(あつ)くありませんでした。
  Hokkaido was not hot.

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.

• この車(くるま)は高(たか)くなかったです。
  This car was not expensive.