As far as the subdivided boxes go, you're doing fine. You're attempting to align your subdivided grid lines towards each vanishing point, and extending them back to the VP to check their alignment is a good way of applying the core principles of the box challenge to this variant.

As to the cylinder challenge, yes - the minor axis line should ideally orient towards one of the vanishing points (the one governing the lines that flow from one end of the cylinder to the other). As to why we draw a line that connects the middle of one plane to the other... Well, the reason for that would be that this gives us an edge that runs towards that vanishing point, and thus gives us a line to align our ellipses to. Of course, we check the "true" minor axes for our ellipses afterwards to see how close/far we were from aligning it as intended, but there's no trick here. A edge going from the center of one end to the other will still run parallel in 3D space to the edges running from the corners of one end to the other, and thus the lines used to represent them will align towards that same vanishing point.

And to your last question, I have two quick points of advice to offer:

• A lot of students will try and create a shopping list of concepts they "need" to learn. Instead of that, I'd leverage the 50% rule to actually produce pieces and use them to identify (whether yourself, through feedback from others, etc) what areas of weakness they reveal. Get into a habit of periods of doing your own work, followed by periods of attacking some of the areas of weakness through courses, followed by more of your own work, etc. That way you can focus on the concepts that are most relevant to you and your goals.

Anyway, before I end up without time to actually critique your work, let's move onto looking at your homework. Fortunately your work is honestly really well done (and that's the main reason I indulged your questions, as I don't expect this critique to be very long on its own). Starting with your form intersections, I'm seeing extremely confident linework, as well as intersection lines that demonstrate great comfort not only with the simpler flat-on-flat intersections, but also with the more complex round-on-flat and round-on-round ones, which students generally struggle with at this point. I won't dwell on it, but I did want to share this diagram - based on what I'm seeing, you already understand everything depicted there, but I didn't want to neglect to provide it, in case it solidified your understanding of some of the things you were doing instinctually.

Continuing onto your object constructions, you've demonstrated an enormous amount of patience here, and have held very strongly to the core principles of the lesson: precision. Precision is often conflated with accuracy, but they're actually two different things (at least insofar as I use the terms here). Where accuracy speaks to how close you were to executing the mark you intended to, precision actually has nothing to do with putting the mark down on the page. It's about the steps you take beforehand to declare those intentions.

So for example, if we look at the ghosting method, when going through the planning phase of a straight line, we can place a start/end point down. This increases the precision of our drawing, by declaring what we intend to do. From there the mark may miss those points, or it may nail them, it may overshoot, or whatever else - but prior to any of that, we have declared our intent, explaining our thought process, and in so doing, ensuring that we ourselves are acting on that clearly defined intent, rather than just putting marks down and then figuring things out as we go.

In our constructions here, we build up precision primarily through the use of the subdivisions, something you've applied to great effect. These allow us to meaningfully study the proportions of our intended object in two dimensions with an orthographic study, then apply those same proportions to the object in three dimensions.

I'm also very happy to see that you took the orthographic studies/plans introduced in the computer mouse demo and leveraged them far more effectively than was presented there. This is something we get into much more in Lesson 7, and I've taken more recently to explaining how to use subdivision on the orthographic planes to identify specific positioning for each major landmark, but you're already doing this exceptionally well. So, I'm going to focus the remainder of my time not on gushing over the quality of your results, but rather on a couple areas where things could be done a bit differently, or where you can adjust your approach to yield some benefits.

Firstly, on your metronome, the box you started with is admittedly kind of wonky. This happens on occasion, even for those who include a lot of the freely rotated boxes with line extensions from the box challenge into their warmups, but the fact that in this lesson we're allowed to use a ruler gives us an advantage that can only be used if we're aware of it. A ruler effectively gives us an extension of our line, a sense of how it continues on and converges with other lines in the construction, without having to actually draw the line all the way down its length. Meaning, if we pay attention, we can see how our line is actually converging with others, if we follow along the edge of the ruler. This can help us identify cases where our convergences are off ahead of time, and if we're attentive, we can avoid boxes that are especially off the mark.

Secondly, for your huion stylus, the way you approached this one was fine - great, even - but I have one recommendation that would have made it a lot simpler. Instead of constructing the whole thing in a single box, you could use a big, flat box for the stylus holder, and then extrude a much narrower box from its center to contain the length of the stylus. This would save you from a lot of extra subdivision, without any loss in precision.

And lastly, be careful with your filled areas of solid black. Reserve them only for cast shadows, (so for example if we look at the metronome again, you did that for the long buttons, but if we look at the little coloured lights along the top, you placed the solid black along the opposite side, suggesting that they're meant to convey the side of the form, or something else. There's also what appears to be a cast shadow along the inside of the display, but the display itself has not been constructed as being inset into the surface - so while the cast shadow matches reality, the structure present in the construction alone does not support it.

Also, remember that when you're using filled black areas to define the side planes of forms (like the inside of the huion pen holder), this is more akin to form shading (since we're defining a surface as being lighter or darker based on its orientation), which as discussed here is not something to be used in this course.

And that about covers it. I'll mark this lesson as complete. Keep up the great work. Hm, in retrospect the critique didn't end up being that short...