How to Conjugate Japanese Na-Adjectives ~ Learn Japanese with Toby ~ な形容詞

Conjugating Japanese い-Adjectives: Easy Guide for Beginners

Na-Adjectives also have two functions: (1) they can be predicates, and (2) they can modify nouns. In this video, we’ll focus on predicative conjugation. As you see, when na-adjectives function as predicates, な(na) is dropped.
In English, adjectives don’t need to be conjugated. You just change the form of the verb 'be' according to the tense and subject. In Japanese, however, “na” is omitted with na-adjectives, and the copula such as “desu” is conjugated.

e.g. 元気な(genkina) =energetic, healthy
Polite form, non-past, affirmative is 元気です(genki desu)　negative is 元気ではありません(genki dewa arimasen)　元気じゃありません(genki ja arimasen)
Polite form, past, affirmative is 元気でした(genki deshita)　negative is 元気ではありませんでした(genki dewa arimasendeshita)　元気じゃありませんでした(genki ja arimasendeshita)
Plain form, non-past, affirmative is 元気だ(genki da)　negative is 元気ではない(genki dewa nai)　元気じゃない(genki ja nai)
Plain form, past, affirmative is 元気だった(genki datta)　negative is 元気ではなかった(genki dewa nakatta)　元気じゃなかった(genki ja nakatta)
There are two ways to say the negative form.
じゃ(ja) is a colloquial contraction of では(dewa), and では(dewa) is more polite and formal than じゃ(ja). では(dewa) is often used in writing or in formal situations, while じゃ(ja) is commonly used in everyday conversation or among close acquaintances.