Final answer:
Headlights are legally required to be used half an hour before sunset to half an hour after sunrise, during heavy rain or fog, and in any conditions where visibility is significantly reduced, though laws can vary by location.
Step-by-step explanation:
By law, you are generally required to use your headlights on your car during certain times to ensure visibility and safety on the road. Specifically, headlights should be used from half an hour before sunset to half an hour after sunrise, during any kind of heavy rain or fog, and whenever visibility is sufficiently reduced that objects on the road are not clearly visible from a distance of 500 feet. These laws can vary by jurisdiction, so it's important to check local regulations. However, the most common answer to when headlights are legally required to be used would be: half an hour before sunset to half an hour after sunrise, during heavy rain or fog, and during nighttime when visibility is low.