理由を知りたい

want to know the reason

→ 理由
phrase Uncommon #12,706
want to know the reason 理由 means “reason,” を marks it as the object, and 知りたい means “want to know.”
JA: 彼が来なかった理由を知りたい。
EN: I want to know why he didn’t come.

傷の理由を知りたい?

Do you want to know how I got this scar?

— 落下の解剖学 (2023)

出してくれとは言わないからなぜ閉じ込められたのか教えてくれ理由を知りたいんだ

I’m not asking you to let me out, so just tell me why I was locked up. I want to know the reason.

— オールド・ボーイ (2003)

トムがまだ結婚してない理由を知りたいんだ。

I want to know why Tom still hasn't gotten married.

トムとメアリーが今日ここにいない理由を知りたい?

Do you want to know why Tom and Mary aren't here today?

私はその理由を知りたい。

I want to know the reason.