Can dentin be exposed on chewing surface of front tooth?

Yes it can. The chewing surface of front teeth can become chipped, worn or damaged as the result of excessive wear, malocclusion, para functional habits, bruxism (grinding), bulimia, erosion, chemical food damage (lemons, limes, etc) and other reasons. These things can expose dentin. Front teeth are extremely important for proper function. See your dentist to evaluate & determine an appropriate treatment plan.
Yes. Dentin is below enamel. If enamel is gone, say you are a bruxer, then dentin appears. You can google dentin and see a photo also. Hope this helps.