Grammar JLPT Level N5 - Lesson 1

Structure of だ:

Noun + だ
［な］Adjective + だ

Details about だ:

だ is an auxiliary verb in Japanese that acts as an expression of determination, or strong assertion. It simply states that something 'is' a certain way. It is a casual equivalent of です (another auxiliary verb).
だ should always be attached to the end of nouns, or words that can behave like nouns, such as な-Adjectives.

アイスクリームだ。(Noun + だ)
It is ice cream.
お寺(てら)だ。(Noun + だ)
It is a temple.
危険(きけん)だ。(な-Adj + だ)
It is dangerous.
大切(たいせつ)だ。(な-Adj + だ)
It is important.

だ

Usage Notes for だ:

だ should never be attached to the end of い-Adjectives. In these cases, it is considered more correct to attach a polite equivalent, like です.

大(おお)きいだ。
It is big. (Unnatural in Japanese)
大(おお)きいです。
It is big. (Natural in Japanese)

While だ is often thought of as the casual 'form' of です, that is actually not correct. です is a polite speech word. The official 'formal version' of だ is である. We will learn this grammar point a bit later. である can often be seen in articles, and formal writing, while です is more common in polite speech.