He accidentally deleted the file.他不小心删除了那份文件。

The lock prevents the gate from opening accidentally.这把锁能防止大门意外打开。

Did it happen accidentally [=by accident] or intentionally?那是个意外还是有人故意这么做的？

He bumped into her accidentally on purpose. [=he bumped into her on purpose but he tried to make it appear to be an accident]他假装偶然遇到了她。