理由

1. Origins, structure, and basic meaning

• Structure: 理由 is a noun read りゆう, meaning the reason or grounds for an action or state.
• Basic usage: Use 欠席した理由を説明する for explaining a reason and 理由は…からだ for stating that reason in a following clause.
  • 欠席した理由を先生に説明しました。 describes explaining the reason for an absence to the teacher; この町を好きになった理由は、人が親切だからです。 says that the reason for coming to like the city is that its people are kind.

2. Common pitfalls and similar expressions

• 理由を説明する
  • を marks the reason for the absence as the content explained.
• 先生に説明する
  • に marks the teacher as the person receiving the explanation.
• 理由は…からだ
  • は introduces the reason as the topic, and からだ supplies the concrete cause.