Hey taz, @Dan B is correct, you need to determine your light direction first. Also, think about all your shapes as 3 dimensional objects that have a top or bottom, side and front plane. This will help you design your shadow shapes more accurately and help you make things look 3D. As far as soft and hard edges, just think about form shadows as having soft edges and cast shadows as having hard edges and that should get you started. Hope this helps :)

For the first image I would say the nose side plane should be as dark as the left side of the face. It will be a little darker at the eye due to occlusion. You have to think about where the light is coming from and which planes are visible to it. For instance, in the second image if the brow is casting a shadow over the eye, it will also cast one under the nose. The value used is relative to the brightness and angle of the light, so you need to be able to imagine the flat planes of the face's forms and the angle the light hits them. Whether the edge is hard or soft depends on how strong the light source is. Dorien Iten has some great videos on shading that might help?