Very nice sketching style Hamid.
- Your drop shadows are a bit obtrusive. In some sketches they could be narrower with more falloff, and drop shadows in my view work better when they fall behind the object, with a 45 degree lighting.
- In some sketches, like on some spheres, your key light is quite high, it works best at about 45 degrees.
- The sketches with more contrast work better
- Perfect your perspective drawing skills - in some cubes and cylinders it looks quite off.
I would like to see some sketches where you work with cold vs. warm tones for ground vs. sky reflections and shadows. The two color effect will create much more vibrance and pop to your sketches.