If your vehicle is showing car won't move forward but the engine revs while in gear, here are the most likely causes and the parts you'll need to fix each one.
Automatic transmissions rely on fluid pressure to engage gears. If the fluid is extremely low due to a leak or being underfilled during a recent service, the transmission cannot build enough pressure to move the vehicle.
On manual transmission vehicles, the clutch friction disc wears down over time. When it is completely worn out, it slips freely against the flywheel instead of transferring engine power to the wheels.
If a CV axle snaps or pops out of the transmission (often due to not being fully seated during recent suspension work), the differential will send all power to the broken side, leaving the car unable to move.
Electronic shift solenoids direct fluid to engage specific gears. If a critical solenoid fails or gets clogged, the transmission may default to neutral and fail to engage any drive gear.
Severe internal wear of the transmission's clutch packs, a broken forward band, or a shattered torque converter will completely sever the transfer of power from the engine to the drivetrain.
In rear-wheel or four-wheel drive vehicles, a snapped driveshaft or completely shattered universal joint physically disconnects the transmission from the rear wheels.