Nature has no black outline only light and shade, you might see that one side of your nose is darker than the other or that there is a shine to you hair where the sunlight touches it. This is light and shade – tonal value.
As a rule, objects in the front of your painting will be sharp in their contrast between light and dark forms. Whereas the further you go back in to your drawing the less of a difference there should be between light and dark.
Try this experiment;
Stand in front of the view you wish to draw/ paint close one eye and half close the other. This should help you see light and shade easier as it removes some of the colour and emphasises the difference between the light and dark areas.
Take a look at the house above; it is as you would first learn to draw a house, a simple flat shape.
However if you look at a house you will see that that it does not have a hard outline. Remember there are no black outlines in nature. By drawing a solid line for the base, the house looks to be sitting on the top of the ground with no firm foundations. In fact if you were to put wheels on the bottom corners it would seem to be able to move or roll down a hill.
To give your house depth and strength you must add tone. If you look at the second figure you will see there are no outlines anywhere. The shapes are created entirely with shading, light and dark which give the house a 3D feel. This allows you to believe that the house has foundations below the surface of the ground and is not simply sitting on top of it waiting to be blown away by a strong wind.
Try to create your drawings in this way.