How do enzymes work?

Answer: Enzymes work as catalysts of biochemical reactions. A catalyst increases or accelerates the rate of a chemical reaction. Digestive enzymes break down proteins, carbohydrates or fats. 

The nice thing about enzymes is that if the particular molecule they work on is not present, the enzyme does nothing. For example, if you take the enzyme lactase and the lactose it works on is not present, the enzyme has no job and does nothing. It will get passed on in the gastrointestinal tract as food protein.