So I'll start out with your questions:

Why and when do you use the branches techniques instead of the sausage technique ? For example for a tail, what will you use ?

The branches exercise is primarily about teaching you how to approach a form that may have lines longer and more complex than you feel you can do in a single stroke. Basically when one hits lesson 3, they may find it too difficult to draw a long stem while maintaining a consistent width throughout it. In order to avoid having it get narrower or wider through its length, the branches technique helps them to build that form up by overlapping many different segments, such that they all feel like they flow together.

So it comes down to whether or not you feel you can keep the width of the tail consistent. At this stage, you may be able to do that, although it depends on how the tail itself is positioned. If it's curled up, that would be harder. If it's relatively straight, or arcing slightly, you may be able to draw the full length of one side in one stroke.

Why don’t you use entire cube for the feet ?

I do something similar to drawing a box for the feet, but I focus only on drawing the silhouette of the form, simply because when we get into feet, they're usually a very small part of a drawing and I want to avoid crowding it with more lines than is absolutely necessary. You can convey the impression that something is box-like - that is, that something can be separated into many separate planes (a top plane, a side plane, a front plane, etc.) entirely through its silhouette, and for the feet, this becomes useful as it gets the job done without additional lines.

When and why do you draw through or not ? For exemple for a back limb.

For the back limb, what is your up to date technique ? Strait hatch, full black or something else ?

If we're looking at the animal from one side (and if that animal is standing still rather than running), the legs on the other side will usually mirror those on our side. For this reason, it's relatively easy to just mirror the lines of the legs we've actually constructed. The point here is that these back legs aren't really important to me, and while I'm drawing them because they are in fact there, I am not interested in making them feel three dimensional. In fact, I use the straight hatching specifically to flatten them out, so the legs that are closer to me are going to read more clearly, and stand out against them.

If however we're looking at the animal from a different perspective, I would probably fully construct all four legs, or perhaps three out of four legs.

Your questions tend to focus very specifically on what techniques to use when, which suggests that you're looking at these lessons as a formula on how to approach drawing these individual objects - but there is an important reason the lessons are not titled "How to draw ____". These are not exercises focused on specific formulas and recipes to apply to drawing specific things.

Instead, each and every drawing we're doing here is an exercise. It is not about the end result, to have a pretty, detailed drawing to show our friends and family. Each exercise is about learning how to think in three dimensions, how to understand the way in which your forms relate to one another, how they can be combined, etc. The techniques all exist to solve specific kinds of problems, and at the end of the day, you won't necessarily be expected to draw everything with dense, concentrated construction. In these lessons, you are - but if you look at the first three videos in Lesson 0 (newly added as of this February), you'll see that I draw things fluidly and organically, with no construction whatsoever. But what governs how I draw is my underlying grasp of 3D space. That is what you are working to develop now.

So, now that I've written about one critique's worth of answers and explanation, let's get started on your actual critique.

Before I start, I want to make one thing very clear: there are a number of problems I will be focusing on, but there are a lot of strong qualities to your work - specifically your observational skills. There are a lot of things we're going to have to work on as far as applying construction, respecting the 3D and solid nature of the forms we draw, and so on but you are moving in the right direction with these. By sorting out the issues I address below, you'll be well on your way to drawing considerably more believable and solid creatures.

Looking at your organic intersections, these are pretty well done. The forms themselves are drawn in such a way that you're establishing how they interact with one another in 3D space, and you're conveying a believable impression of gravity that causes these forms to slump and sag over one another. One issue I do see however is that your linework is somewhat stiff in places. It isn't extensive, but there is definite hesitation to your linework.

Don't forget that every single mark we draw should be drawn with the ghosting method. The ghosting method itself exists purely to separate the mark making process into individual stages, each with their own responsibilities, so that we can execute our marks without guilt, fear, or hesitation - just pure confidence, purely to maintain a smooth, consistent stroke.

Moving onto your animal constructions, overall there are some things you're approaching reasonably well, and other areas we certainly need to work on. First and foremost though, I think you're pushing detail way too much, way too early. There's a great deal more you can be doing with things like your lions' bodies to capture the subtler muscle forms that are present along their torsos and shoulders and limbs, but you've skipped right past them in order to attempt to capture their fur and manes. Detail, at this stage, is a distraction.

Right now you appear to be thinking in terms of dividing your drawing into two sections: Construction (simple), detail (complex). This is not correct. Construction itself can be very complex, with plenty of additional forms wrapping around the basic structure to help give the impression of various muscle groups, and to capture the more subtle elements of an animal's body.

For example, if you look at this puma construction demo, at step 13 I start adding a lot more individual forms that wrap around the existing structure. This is a section you largely skip in your drawings.

Another point I feel you may not be entirely understanding is that when you put a mark down on the page, it's generally to establish a solid, three dimensional form in the world. These forms you place in the world are real, they're tangible. They have weight, volume and thickness to them.

Looking at this bird's wings, we can see where you drew a shape in for its wing, and then drew the feathers in such a way that the viewer needs to believe some of the form you'd previously drawn is no longer there. Here's what I mean. If you establish a form as being present in the world, you cannot simply ignore that it exists later on. Instead, you need to build on top of it, building additively. We can see a similar mistake here where you've established a solid sausage form, but cut back into its two-dimensional silhouette.

There is such thing as 'subtractive' or 'subtractional' construction, but it still involves respecting the fact that your forms are three dimensional - not just flat shapes on a page - and requires you to actually interact with those forms in three dimensions. As you can see here, I can cut into a ball form by drawing a line along its three dimensional surface, separating it into two separate parts, then asserting to myself that one part is "positive space" and the other part is "negative space" (or empty/void/missing/etc).

One last point I want to stress is that you're frequently forgetting to draw through your ellipses. Don't forget that you should be drawing through each ellipse two full times before lifting your pen (after applying the ghosting method) to keep your ellipses smooth and evenly shaped. An uneven ellipse won't read as a solid, three dimensional form due to the additional complexity introduced by the various wobbles in its lines.

Now, I know that there is plenty more advice to be given, but aside from this critique already being very lengthy, all of the details you've added to your drawings honestly make it difficult to pinpoint specific problems and point them out in a way that'll make sense. As such, I'm going to stop this here and allow you to first focus on the things I've pointed out here, then we'll move forwards. I'll be assigning additional pages for you to do below.

There is one other thing I wanted to mention that you can apply when submitting your next set of drawings. When scanning your work, I believe your scanner is currently set to increase the contrast on your drawings. Often scanners come with "drawing" presets that really try and force the drawing into stark black and white images, and in doing so they get rid of a lot of the subtler and more nuanced aspects to our linework. If your scanner software has other presets - like ones for photos - use those instead, as they'll capture your work more accurately and make it look less harsh.

Also, when doing your work, I strongly recommend that you take a look at the informal demos if you haven't already. There is a lot of additional information there that breaks down how to approach constructional drawing.