When the oral cavity is not cleaned, the calcium in the tooth phalanx will be dissolved by the acid produced by bacteria, and the decalcified spots will be produced at first, when the decalcification occurs, it means the place will be decayed soon, but fortunately, this period is a reversible state, if it can be brushed and fluoride products are used, the decalcified place may be recalcified, but if it is still not brushed, the decalcified place will become decayed soon. If the oral hygiene is not well controlled, tooth decay will start, and it will invade the dentin at the beginning, at this time, the patient may not feel sensitive to hot and cold, so it is difficult for the patient to detect the early tooth decay by himself. 2.Dentin caries When the cavity continues to go down and exceeds the depth of the dentin, it will invade the dentin, then the patient may be sensitive to temperature, at first the patient is only sensitive to cold, when the cold material is removed the pain will be relieved, if the cavity is not controlled at this time, the cavity will slowly approach the nerve, then the patient will be sensitive to heat or pain, when the hot material is removed the pain will be slowly relieved. The pain will be relieved when the hot material is removed. 3.Tooth decay that invades the pulp cavity The tooth decay is not controlled at the early stage, once it exceeds the depth of dentin, it will invade the pulp cavity, the pulp cavity is commonly known as the nerve, the patient will have spontaneous severe pain during the acute inflammation period, even if the hot or cold substance is removed the pain will not be relieved, when the acute period is over, the spontaneous pain symptom will disappear, the tooth will enter the chronic inflammation state, at this time because the spontaneous severe pain disappears Many people think that the tooth is fine, so they miss the opportunity to treat it. 4. Inflammation of the root tip When the inflammation of the tooth pulp is not controlled, it will spread to the tip of the tooth root, some people will start to appear pus, and some people’s inflammation will continue to spread outward to form cellulitis, cellulitis is a very dangerous state, in addition to the teeth need to be treated also need to take oral or injectable antibiotics to control the inflammation concern dental knowledge. 5.Stumpy root When the tooth decay progresses to the final stage, the tooth structure will disintegrate slowly and finally form a stumpy root, and then the tooth will only be extracted.