Tsk tsk. Someone's forgetting the logic games bible!
If I remember correctly, they actually include a section about must be true, can be false, etc. They actually recommend you write "3 must be true, 1 not necess. true" for something like "must be true EXCEPT"
Not that I do this either. I should though, since even if I underline or circle the EXCEPT, I still get confused like Green and start looking for the wrong answer.