No problem.

Looking at your rhino skin, bricks and fugu skin as examples notice how you outline a form that would create a shadow (like the spines or bricks themselves) but don't really create the cast shadow itself, there's not really any shape for the shadow itself to fill. Compare this to the shadows you create for your organic forms in the organic intersections exercise.

I'll include a texture explanation that I've written down below as well which may help provide some clarity.

Explanation

First things first, open up this leaf texture picture, I find leaves are a good example and a texture that people are often drawn to and do incorrectly.

The first thing you may notice is that this image isn't in colour and instead in black and white, this is helpful because people often get distracted by shifts in colour and will try to darken an area in their drawing if the colour happens to be darker. We shouldn't rely on converting images to black and white but it is helpful and something you may want to consider when practicing.

Now if I handed a student this image and told them to use it for their texture exercises there are two typical outcomes I would expect.

The first is that they would draw all of the veins (or many of them if they aren't extremely patient), and this would be an example of focusing on outlines.

The second result that I would expect is that instead of drawing the veins themselves they would either fill the veins in completely with black or they would fill in everything but the veins completely with ink, and this would be them focusing on negative space.

This is where students get a bit confused at the start, they feel like they're looking at the image and drawing what they see but it comes out wrong. I should clarify that it's not necessarily incorrect, there is a time and place when obsessing over small details can be helpful but this is largely an exercise about learning how to imply information and thinking in 3D space. Remember that what we're learning here isn't observational drawing, it's constructional and while observation is definitely a part of the construction method there's an extra step that people tend to neglect in the beginning.

With that in mind let's go over the correct way to tackle these problems, bring up the leaf image (it's here if you closed it) and let's break down what we're working with. What people tend to neglect is that we're trying to think about the 3D space of the image we're observing and drawing. If we look at this leaf the veins are really just long organic forms, or cylinders if they're particularly rigid. The fleshy bit of the leaf could be thought of as either a plane or a thin box, and due to gravity's effect it will likely curve a bit (we don't need to think too much about it curving in this case because we're working so close up, but it's good to think of how the environment can have an effect).

To put it simply, a leaf is just organic forms that are intersecting with each other and a plane/box, just like the forms we practiced with in the form intersections exercise. With all of these forms in mind we can place a light source and depending on it's position and intensity we can create cast shadows much like we did in the organic intersections exercise.

An example of this can actually be seen in the picture itself. Let's just focus on the large main vein as well as the branching vein on the left. You'll notice that if you look along the bottom edge of the main vein there's a cast shadow, and on the right of the branching vein there's a shadow. From this information we can assume that the light affecting the leaf the most is somewhere to the upper left of the image and it's creating the cast shadows we want to draw.

This is an example of what drawing cast shadows might look like, it comes up early on in lesson 3 when Uncomfortable shows the process of drawing a leaf. You can see that he's not actually drawing the veins themselves, and instead just implying that they exist by creating cast shadows. Keep in mind that this leaf is fairly evenly lit and the point of the texture exercise is to work with light gradients but regardless it's a good example of what we're trying to achieve. There are times where capturing shadows doesn't immediately give the impression of what you're attempting to draw but it's just a single tool that you can use. You may not think that drawn leaf looks like a leaf, but if it was green, had a stem, and was attached to something that resembled a plant your brain would begin fill in the gaps until it goes "oh that's a leaf".

Implying information helps both the creator as well as the viewer, it saves the creator time from having to obsessively capture every tiny detail and it prevents the image from becoming too visually noisy and overwhelming for the viewer.

Craig Mullins is a painter whose work I appreciate a lot and I feel does an amazing job of implying information through shadow, colour and brush strokes. I highly recommend looking up some of his work if you'd like some examples of just how powerful implied information can be.

Hope this all helped, if it didn't feel free to ask any more specific questions that you feel may help provide you with a bit more clarity.

Also keep in mind this is all just something to keep in mind as you move forward, texture is difficult and we don't expect you to nail it first try, if you're actively focusing on cast shadows you'll continue to see improvement as you progress through the course.