This is the 8th most frequent Japanese word.
だ
"だ" is the casual copula in Japanese, meaning "is" or "to be."
Here, 'だ' is used as the plain form of the copula, stating the identity or nature of the subject.
彼は学生だ。
He is a student.
This usage of 'だ' indicates a direct and plain-form statement about the current condition.
今日はいい天気だ。
Today is nice weather.
Here, 'だ' functions within a subordinate clause as part of reported speech, maintaining the copula in its plain form.