The reason why I emphasize that you MUST start the Etude with a basic sketch is because you will end up trying to swallow more than you can digest and the resulting effect will be negative. You will end up learning twice as slow. It is better to take it in small fast steps.
First it is apparent from the sketch that there is no ‘focus’ in the picture. In real life, when someone looks at something they always focus into a particular aspect of the drawing. Your brain cannot help but be drawn to elements that are salient. Try looking at some object on your table, you will realise you cannot manage to look at the whole thing but often have your eyes focus on parts of it, and often towards text, light, movement, and hard edges.
So in this sketch I cannot determine what is actually the focus, is it the girl at the table, or is it the buildings in the background. The MORE detail you put into something, the faster the eye focuses onto that element. In this case the first thing I saw was the buildings in the back. Second, the eyes will always move to shapes it can recognise, basically text. So my eyes moved to the cafe sign and then the scribble on the window. Third, I saw the girl but because that area is now under shade, everything is dark with no contrast and the colors are washed out. Meaning this part of the picture has less detail than everything else. If you wanted the girl to be the focus in this picture, then you have managed the complete opposite :V.
I suggest you glance over this article on BGs – Creating Depth in Art and Photography.
You have also used perspective here to create a sense of depth, but it is negated because the buildings in the background are filling the empty space into the distance. The only indication of BG beyond that is the clock tower but at the moment it just looks like a flat facade. In this case it would have been more pertinent to draw a street going off into the distance along with decreasing detail and increasing ‘fog’.
The particulate matter is basically the air and dust (and when you look at the sun or any bright source, you will see a halo around it as well, that is due to Mie scattering). The important point here is Rayleigh, because the blue photons from the sky also bounce in the distance again from multiple Rayleigh scattering events. This is why the further away something is, the more faded and BLUE it becomes. We see this in everyday live and hence simulating this on a drawing has the same effect as conveying depth.
Also detail is an important factor. Art is like engineering. Functionality defines form. And as such there is always a reason why you drew something. In order to emphasize the purpose of the drawing you need to engineer the viewer’s gaze towards the element of interest. If you look at various artworks throughout history you will notice that anything outside the focus of the painting is super obscure and undetailed. This is precisely for that reason, you only need to ‘indicate’ that something is there, not fully define it’s existence.
Colour is also an important factor, the more vibrant something is the more the eye gets attracted to it. In this case the girl is the most dull element in the picture but the buildings and sky are saturated.
Also the secondary diffuse light reflection thing is easy if everything had been done step by step but you are jumping the gun. I cannot go through all the factors in this picture because there is too much to talk about.
Remember to START THE ETUDE FROM THE SKETCH. Don’t just draw 1 sketch, draw multiple small sketches with the shadow/values blotted in. The sketch is the most critical stage to filter out all the flaws otherwise you will end up fixing things the whole time and getting frustrated. When I draw something serious, 80% of the time is just thinking about the concept and what to draw rather than the drawing itself. It takes 5 hours to finish a piece but around 20 hours just to think and draw scribbles. You can tell which pictures I didn’t think hard on because the BGs are flat etc.
A slight correction to the anatomy of her crotch and pelvis region because it is slightly distended. Also the arm position for her left arm is deceiving at the moment so I can’t comment on that until there are extra details showing it’s actual position.
The one major thing that I realised is that this pose looks like it has been engineered to reduce the amount of effort drawing hands and feet. If that is the case then instead of continuing I would suggest doing a hands/feet study once to get over that aversion, otherwise you will end up like ZUN. That way you can draw some more dynamic poses in the future. Also when posing, always use S curves where possible, like twisting the torso or bending her back.