I have a 2008 F150 Lariat and the drivers side window will not go down. It will sound as if it is starting to come down but doesn't move. It was working fine one day and just stopped working.

Assuming you have an electric window, it's probably the switch itself or the electric window motor.

To test the window motor, grasp the window on each side of the glass with the palms of the hand. While someone depresses the down button for the window, try pushing the window down. (give it an assist). If the window goes down the regulator motor is shot. (getting weak)

Most likely to be the main switch on driver's side, since passanger circuit goes through the driver's switch. But to make sure, test for 12vdc at the motor itself, it might've gotten disconnected. If u got 12vdc at the motor, replace motor if no voltage present, replace switch at driver's side

I have had the same problem with mine, actually all four windows at times. If you remove the switch assy and take it apart, which is very simple, you just need to clean the contacts and reassemble. This is a very common problem that usually requires not part replacement and takes about 20 minutes to perform