Yes. If the dentin of your tooth is exposed it is important that it is brushed but use a soft bristle brush. The dentin is soft and more succeptable to decay. More importantly, however, is the fact that you likely need to see your dentist and have the dentin covered with a filling to prevent further loss of tooth structure.
Yes. You should definitely continue brushing. You may want to have your dental hygienist or dentist observe how you brush to make sure you are brushing correctly. She or he may also advise you to use a softer bristled brush and to use a less aggressive technique.
Yes. Dental plaque, which causes tooth decay and gum disease, has a great affinity for the gum line area of the teeth. Keeping this area clean is as important as protecting the exposed dentin. Use an ada approved toothpaste, a soft brush, and a gentle yet thorough motion to accomplish this. Complete the chore by flossing the in-between areas as well.