I don't know how this happened.

我不知道这事怎么发生的。

《牛津词典》

How this happened is not clear.

这是怎么发生的还不清楚。

youdao

No one knows exactly how this happened.

没有人确切地知道为什么会这样。

youdao

And I knowed I could pray now. But I didn't do it straight off, but laid the paper down and set there thinking -- thinking how good it was all this happened so, and how near I come to being lost and going to hell.

VOA: special.2010.03.21

Two ways of measuring how this happened and what difference it made is to realize, to return to what I said earlier, that big state structures involve bureaucracies.

如果要弄懂贵族如何获利,以及贵族获利所引起的改变,我们必须意识到大国必须建立官僚机构,我在前面曾提到过这一点

耶鲁公开课 - 欧洲文明课程节选

How did that change happened, was that always there, were you always looking at how did this thought affect the way we actually live our lives?

这种转变是怎样发生的？,这种转变一直存在吗？还是说您总是在思考,这种思想是怎样影响我们的,生活方式？

普林斯顿公开课 - 人性课程节选

"He had problems explaining how this happened, " Sessions told Robert Siegel about the day-long hearing.

NPR: Sen. Sessions: 'Sad Day' for Gonzales and Agency

"I don't know why and how this happened, " said Martin Kind, the chairman of Hannover 96.

CNN: German fans mourn Enke 'suicide'

Congress should convene an oversight committee on how this happened and what it means for drug safety.

FORBES: A Drug Recall That Should Frighten Us All About The FDA