だ

だ is the casual version of です: 学生だ, "(I'm) a student". Between friends it's normal, and in writing that isn't talking to a reader directly (novels, manga narration, news articles) it's the standard.

A bare だ at the end of a sentence can sound blunt or matter-of-fact, so in conversation you'll often hear it softened with よ or ね: 学生だよ. In casual questions it usually drops out: 学生? not 学生だ?

One rule to remember early: だ goes after nouns and な-adjectives only. い-adjectives don't need it, so 高い alone means "it's expensive", and 高いだ is wrong.