Assuming that it is a standard transmission (clutch type) the clutch isn't releasing all the way. There could be several causes depending on what type system you have. Hydraulic, it could be air in system or free play adjustment on master cyl. not proper or clutch pressure plate or disk just plain no good. Solid linkeage, adjustment not right or clutch components no good. Cable type, out of adjustment or clutch components no good again. On all types, release brng. fork breaking and not allowing full release, fire wall flex, or pedal assembly problems.
2004 citreon relay gear selection is very hard 1st and reverse very hard
because there is no further GEAR in reverse.
Check the fluid in the clutch master cyl if it is hydraulic.
Reverse is next to first. Lift and push hard left.
If a transmission for a car becomes stuck between 1st and 2nd gear the fluid should be changed. Old fluid in a transmission can cause hard shifting and stuck gears.
respect
Have you ever changed the fluid and filter in the transmission? If not, that's the first thing you should try.
Transmission is going out. Reverse is always the 1st one to go.
Sometimes it's linkaage (cheap to fix). Sometimes its the tranny ($$$$).
Who's car? if you mean why does a car not reverse sometimes, its probably due to failure in the gears.
Chemical changes are hard to reverse because they involve the formation of new substances with different chemical properties than the original substances. This typically requires energy input or a specific reaction pathway to revert back to the original substances, making the process difficult to reverse.
Depends on what it is. Sometimes the reverse is true.