I'm going to add to what seik said, as he didn't mention much about the cylinder themselves:

Starting with your normal cylinders, you're doing a good job understanding the basic concepts, as the cylinder gets away from the viewer, it will get smaller, but the ellipses will also get wider as they turn more towards the viewer. Now, these two things aren't unrelated to each other; you won't see a cylinder that gets only ever so slightly smaller to have a degree shift between the two ends very big. If the reduction on the scale of the cylinder is big, then the change of degrees between ellipses will follow along and get bigger as well.

As for the accuracy of the ellipses, don't forget that anything you can do to improve your accuracy in the page works; you can add little dots to mark the scale of the ellipses you want to draw, which will make it easier to ghost and not lose track of what you're trying to achieve.

Now into the cylinders in boxes. I think you've done a good job here as well, but I'm going to cover what I think that can help you improve:

When drawing the axis of the cylinder, don't forget that that axis is another line that will be parallel to the lines of the box, I tried explaining it here. Basically, as the axis line will be parallel to another 4 lines on the box, you can use them as a guide to draw your axis guide; you have another thing more to think about, if you do that instead of just trying to join the 2 dots, you'll get better results.

Another similar thing here as well which might help get better results. You can use all of these things to plan every mark you do, ellipses, lines of the box etc, to get them more accurate.

And I don't have much more to say; your boxes end up pretty good, by the end of the challenge you're making consistently equilateral boxes, your marks are confident, and your ellipses end up pretty good too, focusing on confidence as you should be doing. Great job overall!

Good luck with lesson 6!