Why is it hard to shift gears with a manual transmission?

hard to get into first gear and reversegear not shifting in manual car
what does it mean when your gears are hard to shift?
manual transmission shifting hard from 1st to 2nd
manual transmission hard to shift into first gear
manual gearbox problems
how to fix stiff gears
manual transmission hard to shift when warm

Hard, and sometimes impossible shifting into first and reverse is caused by the clutch disc not moving away from the spinning flywheel. This is commonly