Shades of the objects are a whole unity that should be studied. Generally, it is not only the shadow, but you should know exactly where to put them. For that purpose, you should learn some things and put some effort in practicing and trying new things. First of all, you should have the source of the light. After you determine that spot, you should draw lines and connect the edges of the objects. Afterwards, you should draw the shadow between the intermingled lines.
Shades of the Objects And the Important Angles
See more elaborate examples on the pictures and the link below.