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to make certain that someone knows the real facts about a situation
确保(某人)了解实情;使(某人)了解真相
Don't worry, I set him straight (on this matter).
不用担心，我(把这件事)跟他说清楚了。
He thought I was a doctor of medicine, so I put him straight and told him I was a doctor of philosophy.
他以为我是个医学博士，所以我就把话说清楚，告诉他我是个哲学博士。