Toothpaste prevents tooth decay by :

Tooth decay is primarily caused by the action of bacteria in the mouth, which break down food particles and sugars to produce acids. When the pH level in the mouth falls below 5.5, these acids begin to corrode the tooth enamel through a process called demineralization, leading to cavities.

Toothpastes are generally alkaline (basic) in nature. Their primary chemical function in preventing decay is to neutralize the excess acidity produced by bacterial fermentation. By neutralizing these acids, toothpaste helps maintain a balanced pH in the oral cavity, thereby preventing the erosion of enamel. While ingredients like fluoride also assist in remineralizing and strengthening the teeth, the fundamental mechanism for immediate acid control is the neutralization of the acidic environment in the mouth.