When are u supposed to use a nasopharyngeal airway?

All the time: Nasopharynx is part of the upper airway tube -narrow passage just behind the nose.

See answer: A nasopharyngeal airway is used to hold the palate forward and maintain the air passage from the nose into the throat. It's most commonly employed after surgery as the patient is wakening who is having a hard time breathing through the nose, when there is significant palate swelling making nasal breathing difficult, or the unconscious patient who is breathing spontaneously but is needing better no.