Wrong selection gear shift lever
Excessive end float of gear
Wrong clutch engagement
Wornout clutch plate
Synchronizer wornout
Gear teeth wornout
Synchronizing unit stuck
In adequate lubrication
Lubricate the unit
Check and realign
Adjust clutch pedal free play
Re - install springs correctly
Bearing worn out or dry
Synchronizer defective
Synchronizer worn-out
Noise in gear shifting
Gear slip
Gear locked in one gear
Gear clash during shifting