な-adjectives are conjugated into the past tense form by simply using the stem words themselves and adding the appropriate conjugation of です immediately afterwards.
元気でした – was energetic
賑やかじゃない – not lively
綺麗じゃなかった – was not pretty
When two or more な-adjectives are used together to describe the same entity, it is only the finally listed adjective that is conjugated according to usage; all prior adjectives are conjugated into the て Form, which is derived by adding で at the end of the な-adjective’s stem.
効率的で効果的 – efficient and effective
便利で早い – convenient and fast
When we mean to connect な-adjectives that are in their negative conjugation, we use the じゃなくて/ではなくて conjugation of です at the end of all な-adjectives except the last appearing, which is conjugated regularly.
親切じゃなくて同情的じゃなかった – was not kind and not sympathetic
*Notice that we can connect い-adjectives and な-adjectives interchangeably by adhering to their respective guidelines of conjugation.
賑やかじゃなくて楽しくなかった – was not lively and was not fun
*This can be a tricky construct to grasp as there is another grammatical construct, X(ではなくて/じゃなくて)Y which translates to “not X, but Y” and practically has the opposite meaning as the form we see presented here.