The reason why he left is unknown.

他离开的原因未知。

同一单元的其他句子

• The book that you lent me is very interesting.你借给我的那本书非常有趣。
• Do you know where the nearest bank is?你知道最近的银行在哪里吗？
• I will call you when I arrive at the station.我到车站时会给你打电话。
• She is the teacher who taught me English last year.她就是去年教我英语的那位老师。
• Although it was raining, we went for a walk.尽管下雨了，我们还是去散步了。