CLUTCHES
The clutch is the connector and the dis-connector from engine to transmission. If the clutch is not working properly it will do one of two things. (A) Clutch will slip causing higher engine RPM for speed traveled. This condition will most likely create a high clutch pedal engagement point. (B) Car will be hard to shift into gear sometimes caused by actual clutch issues and other times caused by actuation issues. A clutch issue could be a broken clutch disc dampener spring or something as simple as an adjustment or a hydraulic failure.
MANUAL TRANSMISSIONS

The job of the manual transmission is to drive the car in the gear selected. Typical problems with the manual transmissions are hard shifting usually indicated by a grind into gear or popping out of gear selected. Many other complaints of customers are due to noise in the transmissions often times quietest in fourth gear. The typical cause of noise starts with bearings and seals for the transmission. For grinding or popping out of gear problems it can get more involved (i.e. worn shift forks, sliders, syncros, or other miscellaneous parts).