As an adjective:
* The auxiliary engine kicked in when the main engine failed. (Describing a secondary or supporting engine)
* The auxiliary nurse assisted the doctor with the surgery. (Describing a nurse who provides support)
* The auxiliary verb "do" is used in questions and negatives. (Describing a verb that helps another verb)
As a noun:
* The pilot activated the auxiliary fuel tanks. (Describing a secondary or backup fuel supply)
* The auxiliary fire alarm sounded, alerting everyone to evacuate. (Describing a secondary alarm system)
* The auxiliary police force provided additional security at the event. (Describing a supporting police force)
In a more complex sentence:
* The auxiliary power unit is used to provide electricity to the aircraft while on the ground. (Explaining the function of an auxiliary system)
The specific sentence you use will depend on the context and what you are trying to communicate.