I have a 2012 Hyundai Sonata. I purchased it used I’ve had this car for 2 years, and every month I had to replace the fuse for the brake lights and, the headlights on the left side doesn’t work even with the bulb replaced . I’ve noticed other Hyundai sonatas with the same problem.

 

Do both the hi beam and low beam for the left headlamp not work ?
Are you installing the correct size fuse for the stop lamps?

 

the high beam works only. And I had the parts store and even a mechanic put the fuse in, and it still doesn’t work.

 

Since the hi beam works , it would rule out a ground problem.
I don't know why your mechanic didn't go any further to determine the cause, but this is what I would do.
Trace back to where the other headlamp gets it's power for the low beam and see if the lights share the same power supply. If they do,then there is a break in the wire to the L/S low beam. If they have a separate power supply to each lamp for low beam then the power source needs to be confirmed and then trace on down to the L/F headlamp and see where the power is lost.. This is not a complicated procedure , but don't know if the mechanic didn't want to get involved or you didn't want to pay to have the problem corrected.