Harry had never been in the habit of studying ahead, unless Hermione dragged him into it.
In their second and third years, Hermione had indeed made Harry and Ron prepare for their lessons, but by their fourth year, she had all but stopped.
For one thing, Harry's and Ron's grades had become good enough that they no longer needed Hermione to guide them through the material beforehand.
After all, Hermione only wanted to help those who were falling behind, not force another academic prodigy into existence.
For another, Harry and Ron both had girlfriends now. Although both Lavender and Qiu were absolutely certain that nothing would ever happen between them and Hermione, Hermione was a considerate girl and still kept a little distance from the boys.
As a result, Lavender and Hermione, who had previously been at odds over their views, got along much better after Lavender started dating Ron. As for Hermione and Qiu, they had been on good terms ever since Harry and Qiu got together.
A top student and an academic prodigy did have quite a bit in common.
So, after their fourth year, Harry and Ron barely studied ahead anymore. At most, they gave the material a quick glance the day before class.
Something Ron had said back then was still in use today, and many students had come to regard it as a classic: "Studying ahead only diminishes the sense of surprise we get when learning something new!"
Harry and Ron, who had not prepared their textbook material in advance since their fourth year, still made no deliberate effort to do so even after entering the advanced class in their sixth year.
It was only because they had acquired that old book that the two of them discovered there was actually a method for brewing