Starting with your organic forms with contour lines, these are looking good. You're sticking pretty closely to the characteristics of simple sausages, and your contour lines are looking confident and accurate.

You mentioned that your ellipses generally come out fainter because they're drawn more confidently than the other marks - one alternative thing to consider is that when you draw them, you may be holding the pen at a lower angle. Fineliners tend to have poorer ink flow when they're held at too low of an angle - how low depends on the brand, but you may want to try holding the pen a bit straighter - not necessarily perpendicular to the page, but higher for sure. Generally a pen in a decent state will make a dark, rich mark even when drawing quickly.

Moving onto your insect constructions, aside from a couple of issues, you're largely doing a pretty good job. While you say you're struggling with understanding how your forms exist in 3D space - and I am sure you do still feel confused - I make a point of listening more to what the drawings themselves tell me about what a student does or doesn't grasp. From what I can see here, your overall understanding of how these forms interact with one another in 3D space is coming along fairly well.

Now there are some places where you undermine yourself a little bit, but compared to most students at this stage it's fairly minimal. This kind of issue comes up in examples like this insect where because those original masses were drawn so faintly, you ended up tracing back over them, effectively redefining the silhouettes of those forms. Sometimes this resulted in just minor alterations, in other cases you ended up cutting across the silhouette of the original masses. Either way, this results in some problems in the resulting construction.

Because we're drawing on a flat piece of paper, we have a lot of freedom to make whatever marks we choose - it just so happens that the majority of those marks will contradict the illusion you're trying to create and remind the viewer that they're just looking at a series of lines on a flat piece of paper. In order to avoid this and stick only to the marks that reinforce the illusion we're creating, we can force ourselves to adhere to certain rules as we build up our constructions. Rules that respect the solidity of our construction.

For example - once you've put a form down on the page, do not attempt to alter its silhouette. Its silhouette is just a shape on the page which represents the form we're drawing, but its connection to that form is entirely based on its current shape. If you change that shape, you won't alter the form it represents - you'll just break the connection, leaving yourself with a flat shape. We can see this most easily in this example of what happens when we cut back into the silhouette of a form.

Instead, whenever we want to build upon our construction or change something, we can do so by introducing new 3D forms to the structure, and by establishing how those forms either connect or relate to what's already present in our 3D scene. We can do this either by defining the intersection between them with contour lines (like in lesson 2's form intersections exercise), or by wrapping the silhouette of the new form around the existing structure as shown here.

You can see this in practice in this beetle horn demo, as well as in this ant head demo.

In your case though, the problem really does come from the fact that those initial masses are drawn more faintly, functionally changing how you view and interact with them in some of these constructions. I believe that if you're able to solve that, you'll more comfortably and consistently be able to interact with those masses as though they are solid, 3D forms.

Moving forwards, you're clearly making an effort to use the sausage method to construct your insects' legs, in that you're sticking to simple sausages structures. I did however notice that in many cases, you aren't adding the contour line at their joints to help define how those segments connect/relate to one another. Also, keep in mind that the sausage method is just intended to lay down a base structure/armature - it's not the entire leg. You can, and should, build on top of it, identifying additional, smaller forms from your reference image that you can wrap around that structure as shown here, here, in this ant leg, and even here in the context of a dog's leg (because this technique is still to be used throughout the next lesson as well).

Aside from those points, your work is coming along quite well, and a number of these - including the coconut crab, dragonfly and stag beetle - demonstrate a really well developing grasp of 3D space. While you may feel less confident in these things, students have a tendency to underestimate and undersell themselves, but their drawings don't lie. I'll go ahead and mark this lesson as complete.