You can't take the exam unless you're registered for the class, and we need to make sure we can verify that.

否则你是不能参加这次考试的,除非你注册了这门课，而我们需要确定你能证实这一点。

麻省理工公开课 - 化学原理课程节选

He may have even attended a Quaker meeting, but we're not sure about that.

他甚至可能参加过贵格会议,但我们对此不能确定

耶鲁公开课 - 欧洲文明课程节选