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It's been a while since I've read the first Harry Potter book but I can't help noticing that it is generally accepted that the DADA job is cursed and no teacher lasts longer than one year. In fact, Dumbledore confirms that in The Half-Blood Prince when viewing the memory of Tom Riddle asking for the DADA job, suggesting that after he was rejected, he immediately cursed the position and from then on, no professor has lasted longer than one year.
If so, not only why has nothing been done about it- because hey all books have plot holes, but more importantly, when Harry meets Quirrell in the Leaky Cauldron with Hagrid, Hagrid says that Quirrell took a year off to get some first hand experience, but that implies that he was the previous DADA teacher.
Hagrid also mentions that he is 'always trembling' and is 'scared of the students, scared of his own subject' but if he went to the Black Forest and was possessed by Voldemort in 1990 and has come back to teach in 1991 when Harry joins Hogwarts, then there hasn't been any time for him to be faced with any students or any opportunities for anyone, least of all Hagrid, who has no reason to be in the castle at all, to see how he interacts with his subject.
This suggests Quirrell's attitude predates his 1991, which can't be true since there is only a small window between the end of his sabbatical and the beginning of term. This also fails to answer how Quirrell has miraculously survived the curse for at least two years.
I have read of Harry Potter Wiki that Quirrell was the old Muggle Studies teacher but then switched to DADA upon his return from sabbatical. But that also doesn't explain why he would take a sabbatical to 'get some first hand experience' in the Black Forest if he was a Muggle Studies professor.
So my question is, how was Hagrid able to provide such a comprehensive description of Quirrell, if he had only begun his DADA job, and if he had been a teacher for longer- how did he avoid the curse?