Grammar JLPT Level N5 - Lesson 12

Structure of な-Adjective + だ (Predicate):

［な］Adjective + (だ)

Examples:
静(しず)か + だ
元気(げんき) + だ

Details about な-Adjective + だ (Predicate):

A predicate is a word that describes the topic/subject of a sentence. The Japanese equivalent of this is a 述語(じゅつご), which literally means 'statement word'. As a predicate, な-Adjectives can appear by themselves (when the word they are describing does not need to be stated), or they may be used at the end of a sentence.
It is important to remember that all な-Adjectives can also function as nouns, and therefore tend to sound more like a noun than an adjective, when used as a predicate.

素敵(すてき)。
It's wonderful.
彼氏(かれし)は素敵(すてき)です。
My boyfriend is lovely.

When there are no other words apart from the adjective itself, it comes across similarly to 'It's pretty', or 'It's hated', and similar expressions in English. We just have to guess what the 'it' is from context.

な-Adjective + だ (Predicate)

Usage Notes for な-Adjective + だ (Predicate):

Unlike い-Adjectives, な-Adjectives change form depending on where they are in the sentence. At the end of a sentence, they will always be followed by だ, or です (except in casual conversation where だ is omitted). However, when used before the word they are describing, they will always be followed by な. Hence the name な-Adjectives.