い-adjectives and な-adjectives
Japanese adjectives take different forms before nouns and at sentence endings. Watch for な-adjectives ending in い, such as きれい, and the special changes of いい.
Forms before a noun
An い-adjective goes directly before a noun, as in 新しい本 (a new book). A な-adjective takes な, as in 静かな部屋 (a quiet room). Notice how the adjective connects to the noun.
きれい is a な-adjective, giving きれいな部屋. 有名 likewise gives 有名な人 (a famous person). Do not classify an adjective from its last written character alone; check its dictionary label.
Make negatives and past forms
高い changes to 高くない, 高かった, and 高くなかった. The polite affirmative past is 高かったです, not 高いでした. The い-adjective's own ending carries the past marking.
静かです changes to 静かではありません, 静かでした, and 静かではありませんでした. Conversational forms include 静かじゃないです and 静かじゃなかったです. Do not apply the い-adjective's く pattern to a な-adjective.
Check いい and linking forms
The negative of いい is よくない and its past is よかった, not いくない or いかった. Thinking of these forms as based on よい makes the pattern easier to organize.
To connect descriptions, an い-adjective takes くて, as in 安くて便利です (cheap and convenient). A な-adjective takes で, as in 静かで広いです (quiet and spacious). The final predicate determines the polite ending.
Check: Which means 'It was not good': いくなかったです or よくなかったです?
Show the answer and why
よくなかったです is correct. The inflected forms of いい are based on よい: よくない, よかった, and よくなかった.