〜かった
was (い-adjective)
い-adjective − い + かった (polite: かったです)
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The past of an い-adjective: drop い, add かった. 楽しい → 楽しかった, "it was fun"; 寒い → 寒かった, "it was cold". Politely, add です: 楽しかったです.
This is where the two kinds of adjective part ways most visibly. Nouns and な-adjectives use でした (静かでした), but い-adjectives never do: 楽しいでした is one of the most common beginner mistakes, and one worth unlearning early.
And again いい is the exception, built on よい: "it was good" is よかった. You'll hear it constantly, as a word of its own: よかった! "Thank goodness!"
Examples
昨日のパーティーは楽しかったです。
Yesterday's party was fun.
昨日はすごく暑かった。
It was really hot yesterday.
先週のテストは難しかったです。
Last week's test was hard.
あの店のケーキ、おいしかったね。
The cake at that place was good, wasn't it?
旅行はとてもよかったです。
The trip was really good.