Shadows are an extreme important element in portraits. Well defined shadows gives a portrait depth and volume. Shadows can vary in forms and hardness.
Cast shadows comes from the light hitting an edge and being defined on another part of the portrait/subject. Imagine light hitting a nose from an upper 30 degree angle. Under the nose on the upper lip and lower part of the chin will a cast shadow appear. Now people have different opinions about how to define cast shadows. Some want them to show soft and some want them hard – by soft and hard I mean the edges of the shadow. So it’s all about personal preference. Personally I like them kinda semi soft.
Form shadows comes from the turning of the face from dark to light. It’s all the areas on the portrait that’s oval. Like a chin will have more or less exposure of light. Its very important to get these shadows addressed. To address these forms you need to master values. As the form of the subject turns more and more towards the light so does the value. The value should go up or lower in scale (depending on how you address it). By defining the lightingning/turning of the form you really master portrait painting on my opinion.