他对离开的原因完全未加隐瞒。

He was quite open about his reasons for leaving.

《牛津词典》

她对自己离开的原因直言不讳。

She was quite explicit about why she had left.

《牛津词典》

不适宜由我说出他离开的原因。

It's not for me to say why he left.

《牛津词典》

I said, "Software for personal computers mom, dad that's why I'm leaving the hallowed halls of Stanford."

我说“软件是用于个人电脑的，这就是我离开斯坦福的原因“

斯坦福公开课 - 微软CEO-Steve.Ballmer谈科技的未来课程节选

the reason why I had to leave was because I wanted to do business.

我离开的原因是我想去做生意。

和纽约的留学生 - SpeakingMax英语口语达人