When tooth decay injures the nerve of the tooth, there will be spontaneous pain, night pain, and sharp pain from hot and cold stimulation. However, accurate judgment needs to be made by the doctor after examination. 1. Spontaneous pain: If there is paroxysmal or spontaneous pain when the tooth is in its natural state or when it is not eating, and the pain is aggravated at night, it means that the tooth decay has injured the nerve of the tooth and led to inflammation of the tooth pulp. 2. Hot and cold stimulation pain: tooth decay injures the nerve, there will be severe pain when eating too hot or too cold food, and the pain lasts for a period of time before it can be relieved, which means that the tooth decay has been closer to the nerve or reached the nerve. The most accurate way to determine whether tooth decay has injured the dental nerve is to go to the hospital in time, after professional physician’s examination combined with clinical manifestations, and then according to the situation of active treatment, in order to facilitate the recovery of the condition.