I'm currently rereading the book and one of the things that always annoyed me about this book was Harry being trained by Snape. The last time Harry had trouble with a spell (the summoning charm in GoF) Hermione recommended the library and studying the theroy of the spell....I just don't get why Harry didn't do that this time...why he didn't pursue any other means of learning the subject....
The main reason I don't understand is because it was Snape... I would figure Harry would want to prevent Snape from gaining access to his mind.
And I totally understand the problem from Snape's point of view...I mean if the dark lord is gaining access to Harry the last thing you want to be seen doing is helping the boy block the dark lord.... Dumbledore should have realized that issue so I am unclear why he placed Snape in such a bad position.