to discover something弄清,确定,查明
The police have so far been unable to ascertain the cause of the explosion.警方到目前为止尚未查明爆炸原因。
to make certain of something确信
[ + that ] I ascertained that no one could overhear us before I told Otto the news.确信没有人能听到我们的谈话之后,我才把这个消息告诉了奥托。