Grammar JLPT Level N5 - Lesson 3

Structure of な-Adjective:

［な］Adjective + だ
［な］Adjective + な + Noun

Details about な-Adjective:

In Japanese, the vast majority of な-Adjectives are originally nouns, and differ from nouns only in that they have their own set of conjugation rules. The dictionary form of a な-Adjective will always be followed by だ. な is only attached when describing another noun.

魚(さかな)は大好(だいす)きだ。
I love fish. (Fish are loved by me)
大好(だいす)きな野菜(やさい)です。
A vegetable that I love.

As we can see from these examples, だ or です is required at the end of a sentence, but な will be required when using a な-Adjective to describe a noun.
な-Adjectives are mostly words of foreign (Chinese/English) origin. This is why they have evolved to require unique conjugation rules.

な-Adjective