Hi! Sorry for the delay. I'll give you my notes and then I'll go to your questions:

• On the fox you made the whole neck-ears form in one stroke. Go step by step, it's better for the mental processes of building up forms. In this case it ends up flattening a bit the form. Is it intersecting with the cranium sphere? Or is it a superficial layer, where the ears are located? Of course we know what you're trying to do, but the point of this is to build on forms, step by step, one by one.

• The hind leg isn't constructed; if you can't see it completely, you can look at other references or make an educated guess based on the leg you can actually see.

• The added masses are where they're supposed to be, but be careful with those corners. Look at the corners in these masses. They're are aligned with the middle point of the sausage, signaling the way in which they wrap around the form. On your hind legs you almost got it, but not quite, so be really careful with those. And you can notice that, in that demo, where the corners are not needed, there ain't any of them.

I highly encourage you to look at this whole submission. Uncomfortable did the critique, so it's better than anything I could tell you, and it deals with a lot of the same issues you have here.

I would also encourage you to make your own archive of corrections, if you don't have one already. I've been saving a lot of random Uncomfortable's corrections on these lessons and it really helps to see different problems and the way they're resolved.

• Your head construction is getting a lot better. Be careful with the eyes still, remember that they're bigger than we can actually see, and that they're covered by an eyelid most of the time, even when it's hard to see it. On your hyena for example, the eyelid doesn't wrap around the eye as it should, instead looking more representational; the muzzle could still be broken into more planes, as it is, it looks a bit flat.

• As for the feet, you've got the right idea, but remember that once you have the basic form, you have to break it down and/or add secondary masses to make the actual form of a foot. Here's another demo from the lesson linked before that deals with feet.

• Don't forget about the intersections. Your legs still look weak, and the intersections between sausages will help with that.

Now to your questions:

• I could not honestly tell you that I have a specific, reliable method for constructing open mouths. What I usually do is establish the muzzle footprint, same as always, and then construct each seccion of the mouth by itself, defining planes, adding masses, etc; joining them only when I'm done with it; always trying to take some measuraments so the opening doesn't get out of hand. I could be wrong though, so you could ask on Discord about that.

Your head looks good, but yeah, more definition on the planes of the muzzle and a more defined form on it would've helped. Also notice that the nose has its own subdivisions. Try to think of the teeth as the claws on Lesson 4; you can revisit those in the informal demos section. So yeah, it looks good, and it's suggesting the right things, but just a bit more definition on the forms would go a long way.

• Try to get the best references you can and avoid the ones where you can't see the things that you need. That being said, you can kind of see what the feet look like on the hyena, you could look at other references as well to make an educated guess of what they could look like; but then again, these are all exercises, try not to make them even more complicated.

So yeah, you're on the right track. Just to prove the point, I'll ask you for a last non-hooved quadruped, draw it big, search for a good reference photo and make sure to check out all the things pointed out here and in the critique by Uncomfortable. That would be the last one, we're kind of grinding on Lesson 5 and we should avoid that :P Anyway, good luck!