1.When shooting a portrait some of the important factors are lighting, but also making sure your subject if is comfortable and relaxed.
2.With broad lighting, the light is hitting the subject broad side of the face, the side with the ear. Short lighting is when the light source is close and the light hits the features of the face while the broad side is in shadow. Both lighting is effective at 3/4 view.
3.The function of the main light is to cast a shadow.
4.The function of the fill light is to cast a bright shadow.
5.The background light simply lights up the background.
6.I would like to know when its appropriate to use a reflector and why use it instead of another light source.
Yes to your answer to number 1. You could have the best lighting technique in the world and if the model looks miserable the photo will be bad.