だ is the plain, casual version of です. It works like “is,” “am,” or “are” when making a statement.
Use だ after nouns and な-adjectives:
• 学生だ（がくせいだ） = “am/is/are a student”
• 元気だ（げんきだ） = “am/is/are well”
Do not use だ after an い-adjective. 寒いだ（さむいだ） is incorrect because 寒い（さむい） already means “is cold.” Just say 寒い.
Tone matters. A bare だ at the end of a sentence can sound firm, blunt, or traditionally masculine, especially by itself.
In casual speech, people often soften it with particles, as in 元気だよ（げんきだよ） or 元気だね（げんきだね）. It is also common to drop だ entirely in casual questions, as in 元気？（げんき？）.
So use だ for plain casual statements, but avoid overusing bare だ if you want to sound natural and not too abrupt.