Explain how a puncture to the thoracic cavity (from a knife wound, for instance) could alter the ability to inhale.

Punctures to the thoracic cavity can cause hemothorax or pneumothorax. Both of them alter the ability to inhale.

Explanation:

In the case of hemothorax, the punctures causes blood to accumulate in the thoracic cavity, which pressures the lung and does not allow it to fully expand.  The visible consequence is the inability to inhale properly.

On the other hand, if the patient presents a pneumothorax, the lung has been punctured, allowing air to flow into the thoracic cavity. Therefore, pressuring the lung and impeding its expansion. As a consequence the patient's ability to inhale is altered.