"다" is a Korean particle that serves various grammatical functions in sentences. It is often used to indicate completeness or totality, similar to the English word "all." For example, when attached to a noun, it can emphasize that the entire group or category is being referred to. In addition to its use for totality, "다" can also function as a verb ending to denote the declarative mood. This means it is used to state facts or opinions. For instance, in the sentence "그는 학생이다" (He is a student), "이다" is a form of "다" that confirms the statement.