So, you press the clutch pedal...
This extends the shaft from the clutch slave cylinder on the transmission, yes? The shaft on the slave cylinder pushes the arm on the clutch release shaft into the transmission, that rotates the clutch fork inside the transmission to push the throwout bearing against the fingers on the pressure plate, releasing the clutch disk to spin freely, allowing the engine and transmission to rotate seperately (like between gear shifts).
The shaft on the slave cylinder should rest snugly against the arm on the exterior of the transmission attached to the shaft that goes into the bell housing, that is also connected internally to pivot and push the throwout bearing against the clutch pressure plate.
If the hydraulics are working, the clutch slave cylinder shaft extending should cause the arm to pivot the shaft that moves the throwout bearing.
If the shaft moves, but the clutch does not release, then you may have problem inside the bell housing with a clutch fork (the part that moves the throwout bearing) not moving, which could be due to a broken set-bolt or the throwout bearing no longer in contact with the clutch fork.
If the clutch slave cylinder shaft doesn't move, then either the master cylinder and/or slave cylinder is kaput, or the system needs to be bled. OR the shaft for the clutch fork that passes into the bell housing is seized. If that were the case, you probably would not be able to depress the clutch pedal.
Finally, depending on how old the clutch is, and whether there is sufficient movement in the slave cylinder, the nuts on the slave cylinder shaft may need to be adjusted to take up the slack in the system, and move the throwout arm further. This would usually be preceeded by the clutch not disengaging completely and grinding gears a lot.
It is a relatively straightforward system, and if everything is moving, but the clutch doesn't disengage, then the problem is likely inside the bell housing.
