I think you should use also hard brushes for these studies for more believable results. Also try to copy (or just analyze) photos of real primitives from gypsum. All those soft reflections, subtle smoothness of edges, balance between crisp and soft, etc. Devil in the details! 
Yes, it really looks weird with the softbrush, I still have to figure out when to use which brush...thanks for the link, I was trying from real life but the values on those photos seem more clearer so I will try that. I read your post after I did my stuff today but I take your words for the next studies. I am not proud of those especially for the little amount that I can do in one day because I am really slow, but I told myself that I will post something even when it sucks 
Good mindset, keep trying!
I'm, not sure if it helps with your problem, but hope it will. 
Blending techniques: Part 1, Part 2, Part 3
Little bit about theory and terminology: Shading Light and Form
Also you could start with flat shapes of all shading elements from Proko's video and then mix them with techniques from CtrlPaint's tutorials. It's good exercise too.
Keeping my eye on this blog, studies and exercises are the quickest ways to improve, and you seem to be doing them. =)
Thank you for the example, @shiv0r. The links are really usefull, @dvlfsh thank you too! That´s really motivating, @stealcase 
Ok so I tried out the smudgetool and the mixerbrush. I found a way to get in the direction I wanted. During rendering I already noticed that the shadow is too low but I kept going because the effect was in that moment more important for me. So it probably doesn´t make a lot of sense but I learnt sth, so I will keep the thing I did wrong in mind for next studies. I think there is a lack of valuedifference too what makes the form less readable, well, next time 
that was the reference I was working from: 
Yeeeeeeeah, already nice progression since first eggs and cylinders which posted above, You on the track! Time to apply these knowledges on REAL artworks, lol. Really proud of you :3
Thank you! but I still struggle a, the cylinder here isn´t as good. I was trying again from real life but that´s when my values are still lacking but yes improvement is the most important ^^
I think this looks better 
another ball, I was finally faster with that one and a WIP because I don´t just draw forms all day... 
So after a long time ago I took a break today from drawing. I was watching Phantastic Beasts in theater, it was really inspiring
so here is just another WIP from the last days: 
Good work on those shapes, keep it up. I really like your last sketches, hope to see the finished version of the last one
I dig the feel of your sketches 
Thank you @Gyruum and @ragamuffin! yes I will finish it, it just will take some time, ´but I will do it
I forgot to post sth yesterday, whoops: 
So iwas concentrating so much on rendering and how the one shadow affects the other form that I totally forgot to do the basic thing before all that: perspective of the two objects. I noticed that after rendering so I did it without rendering on the second picture. 
