"Was that book interesting?"
It depends on whether the adjective is an い adjective or a な adjective.
い adjectives (which end in い, like おもしろい) must drop the い and add かった for the past tense (and use です). So おもしろい becomes おもしろかった.
On the other hand, な adjectives don't change, but use でした instead of です to signal past tense.
Note that な adjectives are adjectives that aren't い adjectives, but some な adjectives end in い, just like い adjectives, so you'll just need to memorise which is which, really.
I feel that they should seperate the katta from the rest of the word, I was confused at first lol