You add 〜いです to make an い-adjective polite. You add 〜かったです to make it polite and past tense.
い-Adjective + ですPrimary
い-Adjective [past] + です
い-Adjective [negative] + です
You add 〜いです to make an い-adjective polite. You add 〜かったです to make it polite and past tense.
この映画はとても面白いです。
This movie is very interesting.
昨日のパーティーは本当に楽しかったです。
The party yesterday was really fun.
この問題はあまり難しくないです。
This problem is not very difficult.
試験の結果は残念ながら良くなかったです。
Unfortunately, the results of the exam were not good.
このレストランは値段が高いですが、味は素晴らしいです。
This restaurant is expensive, but the taste is wonderful.
Adding 'desu' makes your speech polite. It does not change the word's meaning. In casual talk, you just drop 'desu'. Using 'desu' is always polite and correct.
This form is different from the plain form of い-adjectives. You use the plain form with people you know well. You use this polite form with people you do not know well. You also use it with people like your boss.
Watch out: Do not say "takai deshita." This is wrong. You must change the い-adjective itself. Change "takai" to "takakatta." Then add "desu." So, say "takakatta desu." You only use "deshita" with nouns and な-adjectives.
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