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 - 'Interesting' in its base (dictionary form).

面白(おもしろ)かっ - The conjugation form used with た (and several other auxiliary verbs).

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.

Caution

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.