You have done it before, but when you say, "I know calculus, " Sometimes it means you know it, sometimes it means you know of some fellow who does or you met somebody who knows.

你们以前学过,但当你们说,"我懂微积分",有时这表明你们懂,有时表示你知道谁懂,或者你遇到了懂这个的人

耶鲁公开课 - 基础物理课程节选