i have a problem with my brake light. it doesnt light up when i turn my light on at night. but the brake light itself will turn on when i brake.

 

7443 type bulbs are dual filament. So your "low" filament is most likely burned out but the "high" filament used when you brake is not. You will need to change the bulb. I'd change both sides just so you don't have differences in life span and brightness.