Grammar JLPT Level N5 - Lesson 7

Structure of い-Adjective (Past):

［い］Adjective［い］+ かった

Examples
おもしろい + かった
新(あたら)しい + かった

Details about い-Adjective (Past):

In order to create the past form of い-Adjectives, you must add the auxiliary verb た to the 'past' conjunction form of the い-Adjective. The past conjunction form is created by removing the い and replacing it with かっ. This structure is equivalent to 'was' in English.
面白(おもしろ)い - Base (dictionary form)
面白(おもしろ)かっ - Form used with た (and several other auxiliary verbs)

お風呂(ふろ)が温(あたた)かかった。
The bath was lukewarm.
昨日(きのう)の夜(よる)は寒(さむ)かった。
Last night was cold. (Literally 'yesterday's night')

From these examples, we can see how the casual past form is constructed. In modern Japanese, simply adding です after た is considered to be the standard polite past form.

京都(きょうと)は涼(すず)しかったです。
Kyoto was cool. (In relation to temperature)
このケーキは甘(あま)かったです。
This cake was sweet.

い-Adjective (Past)

Usage Notes for い-Adjective (Past):

Although this is considered to be polite by most people, using です after かった is not academically correct Japanese. However, considering how common its usage has become, it may be used in almost any situation confidently.