Your question doesn't give much detail but the car may not go into gear while you're driving (manual only) because the car's speed and engine speed are not suitable for the gear.
E.g. If you're driving at 100km/h (60mph) it could become very difficult to change down to first gear.
The pivot bearing can cause your car from going into gear, once the car is started. The pivot bearing is located in the transmission.
Rolling friction causes a car to stop.
car starts but when i put it in gear it stop
I just put a new battery and alternator in my car, but when i put it in gear or start slowing down to stop..the car die out. Also the brake light is continely on.
Hold down the clutch when sitting still or going too slow for that gear.
some of the classic signs of a tranny going bad is leaking, slipping in and out of gear delayed starts from a stop where the car raises rpms before it takes off and shaking or jerking when its going into gear
Going to the garage to get new brakes before you gear box goes
That would depend on the gear ratio between the engine and the wheels, and in the diameter of the wheels. The transmission of the car lets you change the gear ratio, so in low gear the car may be going 15 MPH, while in high gear the car may be going at 70 MPH.
If it's a newer model car it could be neutral safety switch or torque convert lock out switch. Is your check engine light on?
It's not in the right gear. It means your going too fast for what gear it is set in. If it hums, put it into a higher gear e.g. if it hums when in 3rd gear, put it in 4th and the humming will stop.
That would be your transmission. It may be slipping, or you may be lossing a gear. Have it checked.
The jump is caused by G forces. The car is moving and you stop the car, but you dont stop right away. You keep going until another force acts upon you, which is yourself and gravity. This applies to the car too. First the brakes stop the wheel, then the wheel stops the axles... and so on. This causes the "jumping forward".