The first seems fine. The second is grammatically correct, but it may not be exactly what you want to say. For a start, 'learn a book' isn't quite right. You can't really learn a book - I'd use a parallel expression to the one in your first sentence, and say 'finish studying'. I'd also use 'can' in the second sentence as well, because you're asking if it's possible. You've used a present simple form, which is grammatically correct, but unlikely - I doubt if you are asking if students always finish the book in a month. So the best way of saying it is probably: Do you think that students can finish studying this book in a month?

Hello Kitae Kim. The first sentence is fine. The second sentence should be written this way: Do you think that students can learn this book in a month?