Hello!

Congrats on finishing the challenge! You've definitely improved a lot throughout the 250 boxes.

When only 2 sides of the box are facing the viewer, then the box is in 2 point perspective and there are only 2 VPs. The vertical lines will all be parallel.

Great job drawing big boxes! Your hatching also looks very good!

For the inner corner you can try changing the order you draw the lines so that you aren't drawing the inner corner last. This should give you more freedom with where you place it so you can improve quickly. It's also important to think of the lines as part of a set with a common VP and make a guess from there (instead of thinking of them in pairs). This diagram made by Elodin might help: https://i.imgur.com/8PqQLE0.png

To practice a larger variety of boxes, make sure the vary the angles and the lengths of the lines of your Y. You can also rotate the boxes more so you barely see the third side and draw those. You can also make the foreshortening less dramatic (make the lines converge more slowly) and practice that. Here's some ideas for boxes to draw: https://imgur.com/Kqg6uMX

Good luck! :)