My driver side window wont go down. how can i be sure whats causing the issue?

Doing one now first you need to take the window switches off the door panel then you need a test light check to see if you have power going to switch . Test light will light up if you touch the wire going to drivers side window switch when you click switch for window to go up or down. If no power to switch check fuses if you have power you have to take the door panel off and check window motor for power if no power when you click switch for window to go up or down the switch is bad or bad wire if power you have a bad window motor.

thank you for replying. fr the driver door, the passenger switch works well, only the driver part i'm unsure of. i already took the door panel off. i will get a test light tomorrow and follow your steps.

Most likely it's the motor very seldom the switch goes bad.

suddenly i can see the glass trying to go up. but it doesn't move when i try going down. i think there's power in it... just seem like the down side bad. could it be the switch?

It's the motor because if it was the switch the window would not have moved at all but still check power to motor if you touch the test light to one of the wires going to motor and click the window switch to the down position if it does not light touch the other wire to motor if either one makes the test light light up you have a bad motor if it does not light up its the switch.