What's wrong with my Tail Lights?

[Khoifather]

At night when I turn on the lights, only the right tail light turns on (passenger side) and the left tail light doesn't turn on at all. The left turn signal works and all but it only turns on when I push on the brakes. Why won't it light up like the right tail light and only light up when I push on the brakes? Does that mean the circuit is bad or do I have to replace the light bulb?

 

[Protege5Gurly]

Replace the bulb .. its a dual filament and one prob burned out

Pe@ce

 

[LapisBlueProteg]

bulb