이유다

In a sentence

• 그녀가 늦은 이유다.

  That's the reason she's late.

  In this sentence, "이유다" is used to indicate the cause or reason for her being late.

• 공부를 열심히 해야 하는 이유다.

  This is the reason why we need to study hard.

  Here, "이유다" expresses the justification or motive behind the need to study diligently.

• 행복하게 사는 것이 중요한 이유다.

  This is the reason why living happily is important.

  In this context, "이유다" is used to present the explanation or rationale for the importance of living happily.