Why doesn't Proctor want Mary to go back to court?I have the recording and the book but can't find the answer, can anyone help?

The place you are looking for is Act II, Scene 2.

In that scene, Mary has just come back from spending all day in court.  She is telling the Proctors about what has happened in court that day.  Specifically, she tells them that Sarah Good has confessed to making a deal with Satan and that Goody Osburn will be executed.

Proctor tells Mary not to go back to court because he is appalled at the type of evidence that was used to convict Osburn.  He is also shocked that the court would not realize that Osburn was just old and a bit crazy.

To find more about this, just go to Act II, Scene 2.