I found her letter by accident as I was looking through my files.

我在查看我的文档时 意外地发现了她的信

Helen got into acting purely by accident.

海伦当演员完全出于偶然。

She discovered the problem by accident during a visit to a nearby school.

她在去附近一所学校参观时偶然发现了这个问题。

It happened—whether by accident or design —that the two of them were left alone after all the others had gone.

碰巧—不知是偶然还是有意安排—其他人走后，只剩下了他们两个人。

Success in life doesn't happen by accident.

生活的成功并不会是偶然发生的。

It was hard to know whether it happened by accident or by design.

很难知道这是偶然发生还是有意为之。