I fill out these grammar books like they were puzzles. Why not have fun and learn at the same time? I just finished filling it out for several months. The other grammar books I worked with were so inferior to this one. I am spoiled. I don't think I'll ever find a better one. For one thing, it has very few errors. I only found one and it was insignificant. Most grammar books contain many more. It also was well organized, clear and complete. In short, it was perfect for what I needed. It took me about 2 months to fill out all 420.