The gear is not engaging, even I switched off the engine and put it to neutral and tried to push the vehicle it can't move backwards it's only moving forward. What would be the cause?

 

Is the shift lever on the transmission moving with the shift lever and if so in sync with the selected gear?

 

Yeah I tried that already, the shift leaver is moving.. the gear cable is okay, transaction between the leaver and gearbox is okay but the vehicle can't move backwards or even forward when accelerating on drive or reverse.
When switching off the engine, put gear to neutral and tried to push but it can't move backwards, it's only moving forward.
I think the problem is inside the gearbox but what exactly could it be?

 

Jack up the front of the car one side at a time. With trans in neutral, and try to rotate the tire now off the ground tire in each direction by hand.