What structure closes the entrance to the trachea (the glottis) when you are swallowing food and prevents choking?

A piece of cartilage called the epiglottis.

Hey!  When food enters the lungs instead of the esophagus, it affects breathing and can cause choking and other problems. The structures inside the mouth and neck each have a distinct role in breathing, speaking and swallowing. The swallowing mechanism is a complex process that, when it works properly, moves food to the esophagus for transport to the stomach. Dozens of muscles and nerves work to move food in the right direction. A piece of cartilage called the epiglottis plays a key role in ensuring that food does not enter the windpipe or the lungs. Hope this helps!