Final answer:
The anatomical structure being described here is the epiglottis, which forms an opening into the trachea.
Step-by-step explanation:
The anatomical structure that forms an opening (mouth) into the trachea is the epiglottis.
During swallowing, the epiglottis closes the glottis, which is the opening to the larynx, and food passes into the esophagus instead of the trachea. The epiglottis effectively blocks access to the trachea and bronchi, preventing food from entering the respiratory system.
For example, when you swallow food, it passes through the pharynx into the esophagus and into the stomach, but the epiglottis covers the glottis, ensuring that the food doesn't enter the trachea and cause choking.