I have an old (1992) VW Jetta and it only starts if the transmission is in gear (and of course the clutch is pressed in.) it will not start if the clutch is engaged but the car is NOT in gear. Why is this?
If you turn ignition and nothing happens, the problem is usually in the interlock system. Manual transmission cars, including the Jetta from what I am reading, have a clutch switch or clutch interlock switch. It is basically a cylinder with a rod sticking out of it which is mounted on a bracket in front of the clutch pedal. Pressing the clutch pedal extends or contracts the rod, making it close an electrical circuit which ignition goes through allowing the driver to start the car.
See here for a good diagram of the switch: https://www.yourmechanic.com/article/how-to-replace-a-clutch-switch-by-marvin-sunderland
You may need to get a new switch, or perhaps the bracket to which it is mounted is bent on your car, or maybe the switch has loose fasteners.