だ

だ is the plain, non-polite copula used to state that something is a certain thing. It connects a subject to its identity, category, or property in informal speech. Use だ after nouns and na-adjectives to make a simple assertion. In past tense, だった expresses that something was true. In negative form, ではない (or じゃない in spoken style) says that something is not the case. In casual conversations, you often omit だ with pronouns or in familiar contexts, but in formal settings you would use です.