put/set someone straight

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。