My tooth hurts when i gently scratch it with my nail?

Dentin Sensitivity: You likely brushed away the enamel protection. You should see your dentist to see if you need to restore the tooth. This is not the only possibility so please get it checked. For now make sure you use a soft toothbrush and a toothpaste for sensitive teeth.

Cavity or recession: receding gums will expose new,root surface that is not protected by enamel, this is a result of gum disease or zealous aggressive brushing. Abrasion and or cervical caries can cause the same thing, it is best to see a dentist to get to the bottom of his issue as the recession or abrasion will keep going and cause additional damage to the hole tooth and may be adjacent teeth as well.