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 an 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 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.
While だ is often thought of as the casual ‘form’ of です, that is actually not correct. です is a 丁寧語（ていねいご） word (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.