Final answer:
The digastric muscle helps to open the mouth and depress the mandible. It is one of the suprahyoid muscles responsible for raising the floor of the mouth and the larynx during swallowing.
Step-by-step explanation:
The muscle that helps to open the mouth (depress the mandible) is the digastric muscle.
The digastric muscle has anterior and posterior bellies that work to elevate the hyoid bone and larynx when one swallows, and it also depresses the mandible. It is one of the suprahyoid muscles responsible for raising the floor of the mouth and the larynx during swallowing.