Reasons why a clutch is fitted to a manual transmission system

Are you referring to the braking system or the clutch system? Generally the rule is that a Master Cylinder is at the pedal end of the system and the Slave Cylinder is at the other end doing the work.
The brake Master Cylinder is usually mounted high on the bulk head, (firewall), and the Slave is usually fitted inside each brake Drum. Disc brakes use the piston inside the caliper as the Slave Cylinder.
Clutch systems have the Master Cylinder bolted to the outside of the transmission case and the Slave Cylinder is mounted inside the transmission, to move the clutch mechanism.
To remove parts will depend on which one you are replacing. Some of these parts are reasonably easy to get to, like the brake Master Cylinder and others need a lot of work, like the clutch Slave Cylinder.
Please comment below if you need more help.

Dennis
need to know the make ,model, year of the vehicle as many manual transmission do not have a neutral switch
If it is a car that requires such a switch to start then the switch is normally in the clutch system ( requiring foot on the clutch) but if it is in the box as well ( foot on the clutch and in neutral) it will be in the side of the box in the area of where the selector rail is located
the exact location will be in the workshop manual or ask your local mechanic familiar with your make of vehicle

Transmission can malfunction due to the following reasons:
- Before blaming the transmission, engine tune up be carried out to get the maximum performance and torque.
- Wrong type of transmission oil is used.
- Improper transmission fluid oil level.
- Contaminated transmission fluid.
- Transmission oil leakage.
- Throttle control cable adjustment. (Auto transmission)
- CVT oil calibration. (CVT).
- Clutch and pressure plate and clutch slippage. (Manual transmission).
- Drive shafts.
- Transmission damaged.

i have fitted a new thermostat heat working fine. but temperature gauge not working

Is it automatic or Manual transmission?
If it is an Automatic Transmission, that may be a concern on the computer box for the transmission or the clutch lining or ATF.

If it is a manual transmission, try looking into the friction disc or the clutch lining.

The reason is that the gears are turn with the engine running, like the clutch is not disengaged. So that is what you need to look at. the clutch or clutch pedal linkage/ hydraulic system. One of these is not letting the transmission slow down enough to engage a gear.

the most obvious question is,you have put the friction plate in the right way facing "front"and the spigot bearing is aligned with the clutch and primary shaft.??otherwise i can see no reason why it wont fit.do NOT use any spacers.if it fits, it fits......

This is the automotive equivalent of a ***-change operation. In short, dont do it...
The problem you have is not if the tranny will fit (which it won't as a direct bolt-in replacement), the problem is in everything else that needs to be moded. Your engines computer is not programed to recognize or work with a manual tranny. You would need to modify the console to accept a manual shifter, install a clutch and clutch pedal assembly, plumb the needed fluid systems, and replace the entire wiring harness in the engine bay. In short, if you are asking - this won't go well and even if it did work, the car would not be reliable. By the time you spend everything involved in this project, it would be much cheaper to buy a 95 or 98 Saturn with the transmission that you prefer.

heavy clutch operation ?p reg 1400cc golf.
having fitted a clutch and manual adjustment cable, the operation has detiorated and become heavy is their any reason ? it has only done 4000 miles tops in the year since fitting