Can not shift gears on 1997 Toyota Tacoma

When the truck is off, I can move the gear shift to any gear I want. When the engine is on, I can not shift into any gear. What is the problem ?

2 answers 2 comments
Check the clutch pedal and see if it is softer than normal. If it goes to the floor with little resistance there is a problem with the clutch hydralic system. If the clutch pedal has the ususl resistance the clutch disc may have damage and cause this condition.
Does the transmission grind when the engine is running and you try to put it into gear?
No, there is no grinding noise. If the truck is in gear when I start it, I can not get it out, or it is pretty difficult move to neutral, of that gear. If I start the truck in neutral, then I am unable to move the stick shift into any gear (i.e. reverse, first, second). I noticed that if I start the truck, in first gear, the truck tends to move forward, even though I have the clutch depressed.
Most likely the clutch disk is damaged, but I have seen issues with the wrong fluid in the transmission creating hard shifting.