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.